Catalyst degradation determining method

ABSTRACT

A catalyst degradation determining method includes the steps of: controlling an upstream-of-catalyst air-fuel ratio occurring upstream of a first catalyst to an air-fuel ratio that is rich of a stoichiometric air-fuel ratio so that first and second catalysts store oxygen up to a maximum storage amount of oxygen. The method then includes the steps of controlling the upstream-of-catalyst air-fuel ratio to a first lean air-fuel ratio until an output of a downstream-of-first-catalyst sensor indicates a lean air-fuel ratio, and then to a second lean air-fuel ratio and that has a value that is determined in accordance with an oxidizing-reducing capability index value, until a time point when an output of a downstream-of-second-catalyst air-fuel ratio sensor indicates an air-fuel ratio that is lean.

RELATED APPLICATIONS

This is a Division of application Ser. No. 10/606,375 filed Jun. 26,2003. The entire disclosure of the prior application is herebyincorporated by reference herein in its entirety. The disclosure ofJapanese Patent Application No. 2002-201544 filed on Jul. 10, 2002,including the specification, drawings and abstract is incorporatedherein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to a catalyst degradation determining method fordetermining whether a catalyst disposed in an exhaust passage of aninternal combustion engine has degraded.

2. Description of Related Art

A known three-way catalyst (in this specification, sometimes referred tosimply as “catalyst”) for controlling exhaust gas from an internalcombustion engine is disposed in an exhaust passage of the internalcombustion engine. The three-way catalyst performs the function ofoxidizing unburned components (HC, CO) (oxidizing function), and thefunction of reducing nitrogen oxides (NOx) (reducing function), and isable to substantially remove harmful components, including unburnedcomponents and nitrogen oxides mentioned above, due to the oxidizing andreducing functions.

The efficiency of removal of harmful components by the oxidizing andreducing functions of the catalyst is known to rise as the air-fuelratio of the internal combustion engine approaches a stoichiometricair-fuel ratio. If the air-fuel ratio of the internal combustion engineis kept within a predetermined range (hereinafter, referred to as“window range”) that contains the stoichiometric air-fuel ratio, theremoval efficiency can be kept at or above a predetermined high value.

The three-way catalyst further has the function of oxidizing unburnedcomponents, such as HC, CO, etc., with oxygen released from the catalystif the exhaust gas coming into the three-way catalyst has a fuelrich-side air-fuel ratio (oxygen releasing function), and the functionof storing oxygen released from nitrogen oxides (NOx) if the incomingexhaust gas has a fuel lean-side air-fuel ratio (oxygen storingfunction). Due to the oxygen releasing and storing functions, thethree-way catalyst is able to substantially remove harmful componentsincluding unburned components and nitrogen oxides mentioned above.Therefore, the emissions control capability of the three-way catalystincreases with increases in the maximum value of the amount of oxygenstorable in the three-way catalyst. Hereinafter, the amount of oxygenstorable in the three-way catalyst will be referred to as “oxygenstorage amount”, and the maximum value thereof will be referred to as“maximum oxygen storage amount”.

The catalyst degrades due to heat given to the catalyst or the poisoningby lead, sulfur and the like contained in fuel. As the degradation ofthe catalyst progresses, the maximum oxygen storage amount decreases.Therefore, if the maximum oxygen storage amount of the catalyst isestimated, it becomes possible to determine whether the catalyst hasdegraded on the basis of the estimated maximum oxygen storage amount. Itis to be understood that “storage” used herein means retention of asubstance (solid, liquid, gas molecules) in the form of at least one ofadsorption, adhesion, absorption, trapping, occlusion, and others.

A catalyst degradation degree detecting apparatus disclosed in JapanesePatent Application Laid-Open Publication No. 5-133264 is provided on thebasis of the aforementioned finding. That is, the air-fuel ratio of theinternal combustion engine is forcibly changed from a fuel-lean air-fuelratio to a predetermined fuel-rich air-fuel ratio (or from a fuel-richair-fuel ratio to a predetermined fuel-lean air-fuel ratio). On thebasis of a corresponding change in the output of the air-fuel ratiosensor disposed downstream of the catalyst, the maximum oxygen storageamount of the catalyst is estimated. A degree of degradation of thecatalyst is detected on the basis of the estimated maximum oxygenstorage amount.

More specifically, the disclosed apparatus first sets the oxygen storageamount at the maximum oxygen storage amount by controlling theupstream-of-catalyst air-fuel ratio to a lean-side air-fuel ratio, andthen controls the air-fuel ratio of the catalyst to a predeterminedrich-side air-fuel ratio. By multiplying the time elapsing until theoxygen storage amount of the catalyst reaches “0” and the output of theair-fuel ratio sensor disposed downstream of the catalyst changes to arich side (hereinafter, the time point of the output of the air-fuelratio changing to the rich side will be referred to as “the time ofair-fuel ratio sensor output rich-side switch”) by the amount of oxygenreleased (consumed) in the catalyst per unit time, the apparatusestimates a maximum oxygen storage amount (hereinafter, this estimationmethod is referred to as “rich air-fuel ratio-based estimation method”).In another method, the upstream-of-catalyst air-fuel ratio is controlledto a rich air-fuel ratio so as to set the oxygen storage amount of thecatalyst at “0”. After that, the upstream-of-catalyst air-fuel ratio iscontrolled to a predetermined lean-side air-fuel ratio. By multiplyingthe time elapsing until the oxygen storage amount of the catalystreaches or exceeds the maximum oxygen storage amount and the output ofthe air-fuel ratio sensor disposed downstream of the catalyst changes tothe lean side (hereinafter, the time point of the output of the air-fuelratio changing to the lean side will be referred to as “the time ofair-fuel ratio sensor output lean-side switch”) by the amount of oxygeninflow to the catalyst per unit time, a maximum oxygen storage amount isestimated (hereinafter, this estimation method is referred to as “leanair-fuel ratio-based estimation method”). That is, this apparatusdetermines a maximum oxygen storage amount by using at least a change inthe output of the air-fuel ratio sensor disposed downstream of thecatalyst, and the predetermined lean air-fuel ratio or the predeterminedrich air-fuel ratio in, for example, a case where there is a need toestimate the maximum oxygen storage amount again, or the like.

If the maximum oxygen storage amount of the catalyst is estimated by therich air-fuel ratio-based estimation method, supply of a mixture of thepredetermined rich air-fuel ratio is continued until the aforementionedair-fuel ratio sensor output rich-side switch occurs. In this case, asthe maximum oxygen storage amount of the catalyst is estimated at thetime of the air-fuel ratio sensor output rich-side switch, it is nolonger necessary to keep the upstream-of-catalyst air-fuel ratio at anair-fuel ratio that is rich of the stoichiometric air-fuel ratio. If theupstream-of-catalyst air-fuel ratio is kept rich of the stoichiometricair-fuel ratio immediately after the air-fuel ratio sensor outputrich-side switch, unburned components, such as CO, HC and the like, arereadily discharged since the oxygen storage amount of the catalyst is“0” and the oxygen releasing function of the catalyst is not effective.Therefore, after the air-fuel ratio sensor output rich-side switchoccurs, it is preferable to set the upstream-of-catalyst air-fuel ratioat the stoichiometric air-fuel ratio or set the upstream-of-catalystair-fuel ratio at an air-fuel ratio that is lean of the stoichiometricair-fuel ratio.

However, at the time of air-fuel ratio sensor output rich-side switch, aspace defined by the catalyst and the exhaust passage from the exhaustport of the internal combustion engine to the air-fuel ratio sensordisposed downstream of the catalyst is filled with a gas having apredetermined rich air-fuel ratio. If in this case, the predeterminedrich air-fuel ratio is considerably below a lower limit value of theaforementioned window range, unburned components contained in the gasfilling the aforementioned exhaust passage and the like, that is, CO,HC, etc., are great in amount. Furthermore, since the oxygen releasingfunction of the catalyst is not effective and the efficiency of removalof unburned CO, HC by the oxidizing function of the catalyst is low,large amounts of unburned CO and HC are emitted into the atmosphereimmediately after the air-fuel ratio sensor output rich-side switchoccurs although the upstream-of-catalyst air-fuel ratio immediatelyfollowing the air-fuel ratio sensor output rich-side switch is set atthe stoichiometric air-fuel ratio or an air-fuel ratio lean of thestoichiometric air-fuel ratio.

Therefore, if the maximum oxygen storage amount of the catalyst isestimated by the rich air-fuel ratio-based estimation method, it ispreferable to set the aforementioned rich air-fuel ratio at an air-fuelratio that is rich of the stoichiometric air-fuel ratio and that isequal to or higher than the lower limit value of the window range inorder to lessen the amount of unburned CO, HC emitted immediately afterthe air-fuel ratio sensor output rich-side switch occurs.

Similarly, if the maximum oxygen storage amount of the catalyst isestimated by the lean air-fuel ratio-based estimation method, the supplyof a mixture having the aforementioned predetermined lean air-fuel ratiois continued until the air-fuel ratio sensor output lean-side switchoccurs. In this case, as the maximum oxygen storage amount of thecatalyst is estimated at the time of the air-fuel ratio sensor outputlean-side switch, it is no longer necessary to keep theupstream-of-catalyst air-fuel ratio at an air-fuel ratio that is lean ofthe stoichiometric air-fuel ratio. If the upstream-of-catalyst air-fuelratio is kept lean of the stoichiometric air-fuel ratio immediatelyafter the air-fuel ratio sensor output lean-side switch occurs, nitrogenoxides NOx are likely to be emitted since the oxygen storage amount ofthe catalyst has reached the maximum oxygen storage amount and theoxygen storing function of the catalyst is not effective. Therefore,after the air-fuel ratio sensor output lean-side switch occurs, it ispreferable to set the upstream-of-catalyst air-fuel ratio at thestoichiometric air-fuel ratio, or set the upstream-of-catalyst air-fuelratio at an air-fuel ratio that is rich of the stoichiometric air-fuelratio in, for example, a case where there is a need to estimate themaximum oxygen storage amount again, or the like.

However, at the time of air-fuel ratio sensor output lean-side switch,the space defined by the catalyst and the exhaust passage from theexhaust port of the internal combustion engine to the air-fuel ratiosensor disposed downstream of the catalyst is filled with a gas having apredetermined lean air-fuel ratio. If in this case, the predeterminedlean air-fuel ratio is considerably above an upper limit value of theaforementioned window range, nitrogen oxides NOx contained in thefilling gas are great in amount. Furthermore, since the oxygen storingfunction of the catalyst is not effective and the efficiency of removalof nitrogen oxides NOx by the reducing function of the catalyst is low,a large amount of nitrogen oxides NOx is emitted into the atmosphereimmediately after the air-fuel ratio sensor output lean-side switchoccurs although the upstream-of-catalyst air-fuel ratio immediatelyfollowing the air-fuel ratio sensor output lean-side switch is set atthe stoichiometric air-fuel ratio or an air-fuel ratio that is rich ofthe stoichiometric air-fuel ratio.

Therefore, if the maximum oxygen storage amount of the catalyst isestimated by the lean air-fuel ratio-based estimation method, it ispreferable to set the aforementioned lean air-fuel ratio at an air-fuelratio that is lean of the stoichiometric air-fuel ratio and that isequal to or lower than the higher limit value of the window range inorder to lessen the amount of nitrogen oxides NOx emitted immediatelyafter the air-fuel ratio sensor output lean-side switch occurs.

It is known that as the degradation of the catalyst progresses, themaximum oxygen storage amount decreases, and moreover, the efficiency ofremoval of harmful components by the oxidizing and reducing functions ofthe catalyst with respect to a fixed air-fuel ratio (oxidizing/reducingcapability) decreases, and the window range of the catalyst narrows. Itis also known that the efficiency of removal of harmful components bythe oxidizing and reducing functions of the catalyst with respect to afixed air-fuel ratio and the window range of the catalyst also changedepending on the temperature of the catalyst.

Therefore, if the maximum oxygen storage amount of the catalyst isestimated by the rich air-fuel ratio-based estimation method or the leanair-fuel ratio-based estimation method while using the assumption thatthe window range is constant, and that the predetermined rich air-fuelratio is a constant air-fuel ratio which is within the window range andnear the lower limit value of the window range, or that thepredetermined lean air-fuel ratio is a constant air-fuel ratio which iswithin the window range and near the upper limit value of the windowrange, the following problem may occur. That is, as the degradation ofthe catalyst progresses, the predetermined rich air-fuel ratio or thepredetermined lean air-fuel ratio becomes an air-fuel ratio that isoutside the window range, and it becomes impossible to lessen the amountof harmful components emitted immediately after the switch of the outputof the downstream-of-catalyst air-fuel ratio sensor occurs.

The amount of harmful components emitted immediately after the switch ofthe output of the downstream-of-catalyst air-fuel ratio sensor occurscan be lessened if the predetermined rich air-fuel ratio or thepredetermined lean air-fuel ratio is set beforehand at an air-fuel ratiothat is near the stoichiometric air-fuel ratio. However, this measurecreates a problem of a prolonged time elapsing from the beginning ofcontrol of the upstream-of-catalyst air-fuel ratio to the predeterminedrich air-fuel ratio or lean air-fuel ratio to the switch of the outputof the downstream-of-catalyst air-fuel ratio sensor (i.e., a prolongedperiod results for calculation of the maximum oxygen storage amount ofthe catalyst).

SUMMARY OF THE INVENTION

Accordingly, a catalyst degradation determining method capable ofminimizing the emission of harmful components, and estimating a maximumoxygen storage amount of a catalyst within a relatively short period oftime, and determining whether the catalyst has degraded on the basis ofthe maximum oxygen storage amount is provided as an embodiment of theinvention.

In order to achieve the aforementioned object, a catalyst degradationdetermining method in accordance with a first aspect of the inventionapplied to an emission control apparatus of an internal combustionengine that includes a catalyst disposed in an exhaust passage of theinternal combustion engine, and a downstream-of-catalyst air-fuel ratiosensor disposed in the exhaust passage downstream of the catalyst isconstructed as follows. First, an oxidizing-reducing capability indexvalue that changes in accordance with a degree of an oxidizing-reducingcapability of the catalyst is acquired, and an upstream-of-catalystair-fuel ratio occurring upstream of the catalyst is controlled to anair-fuel ratio that is lean of a stoichiometric air-fuel ratio so thatthe catalyst stores oxygen in the catalyst up to a maximum storageamount of oxygen.

Then, the upstream-of-catalyst air-fuel ratio is controlled to a richair-fuel ratio that is rich of the stoichiometric air-fuel ratio andthat has a value that is determined in accordance with theoxidizing-reducing capability index value, until a time point when anoutput of the downstream-of-catalyst air-fuel ratio sensor indicates anair-fuel ratio that is rich of the stoichiometric air-fuel ratio. Amaximum oxygen storage amount of the catalyst is estimated by takinginto account the value of the rich air-fuel ratio to which theupstream-of-catalyst air-fuel ratio was controlled. That is, the maximumoxygen storage amount of the catalyst is estimated on the basis of theamount of oxygen released (consumed) due to gas of the rich air-fuelratio.

Then, it is determined whether the catalyst has degraded based on theestimated maximum oxygen storage amount of the catalyst. Theaforementioned “oxidizing-reducing capability index value” is preferablya degradation index value that changes in accordance with the degree ofcatalyst degradation or a value that changes in accordance with thecatalyst temperature. It is also preferable that the oxidizing-reducingcapability index value be a value based on the maximum oxygen storageamount of the catalyst estimated by the above-described method. However,these examples are not restrictive. For example, the oxidizing-reducingcapability index value may be a ratio between the length of a locus ofthe output of an air-fuel ratio sensor disposed upstream of the catalystand the length of a locus of the output of the air-fuel ratio sensordisposed downstream of the catalyst (locus ratio).

Therefore, the value of the aforementioned rich air-fuel ratio can bechanged in accordance with the degree of the oxidizing-reducingcapability of the catalyst (e.g., the degree of degradation of thecatalyst) indicated by the oxidizing-reducing capability index value.Furthermore, as mentioned above, the window range of the catalystnarrows as degradation of the catalyst progresses. Therefore, forexample, the aforementioned rich air-fuel ratio can be kept at anair-fuel ratio that is within the window range of the catalyst and thatis near the lower limit value of the window range, regardless of thedegree of degradation of the catalyst.

As a result, the efficiency of removal of unburned components, such asCO, HC, etc., based on the oxidizing function of the catalystimmediately after the time point at which the output of thedownstream-of-catalyst air-fuel ratio sensor indicates an air-fuel ratiothat is rich of the stoichiometric air-fuel ratio is kept at or above apredetermined high efficiency value, and therefore the amount of CO andHC emitted immediately after the aforementioned time point can beminimized. Furthermore, since the aforementioned rich air-fuel ratio isset at an air-fuel ratio that is as remote from the stoichiometricair-fuel ratio as possible, the period for calculating the maximumoxygen storage amount of the catalyst can be reduced, in comparison witha case where the rich air-fuel ratio is pre-set at a rich air-fuel ratiothat is near the stoichiometric air-fuel ratio.

In order to control exhaust gas immediately following a startup of aninternal combustion engine and further improve the emissions controlcapability following a complete engine warm-up, a construction issometimes adopted in which a first catalyst having a relatively smallcapacity, generally termed start converter, is disposed in an exhaustpassage of the internal combustion engine, and a second catalyst havinga relatively large capacity, generally termed under-floor converter, isdisposed in the exhaust passage downstream of the first catalyst. Sincethe first catalyst is disposed at a position closer to an exhaust portof the engine than the position of the second catalyst in thisconstruction, the first catalyst receives relatively high temperatureexhaust gas. Therefore, the first catalyst is warmed up and exhibitsgood emissions control performance within a short period of timefollowing the startup of the engine The second catalyst, on the otherhand, requires a longer time before being warmed up than the firstcatalyst. However, after being warmed up, the second catalyst exhibitsexcellent emissions control performance due to its large capacity.

In accordance with an embodiment of another aspect of the invention,there is provided a catalyst degradation determining method as describedbelow. In a catalyst degradation determining method applied to anemission control apparatus of an internal combustion engine thatincludes: a first catalyst disposed in an exhaust passage of theinternal combustion engine; a downstream-of-first catalyst air-fuelratio sensor disposed in the exhaust passage downstream of the firstcatalyst; a second catalyst disposed in the exhaust passage downstreamof the downstream-of-first catalyst air-fuel ratio sensor; and adownstream-of-second catalyst air-fuel ratio sensor disposed in theexhaust passage downstream of the second catalyst, an oxidizing-reducingcapability index value that changes in accordance with at least one of adegree of an oxidizing-reducing capability of the first catalyst and adegree of an oxidizing-reducing capability of the second catalyst isacquired. Furthermore, an upstream-of-first catalyst air-fuel ratiooccurring upstream of the first catalyst is controlled to an air-fuelratio that is lean of a stoichiometric air-fuel ratio so that the firstcatalyst stores oxygen in the first catalyst up to a maximum oxygenstorage amount of the first catalyst and the second catalyst storesoxygen in the second catalyst up to a limit of possible oxygen storageof the second catalyst.

Then, the upstream-of-first catalyst air-fuel ratio is controlled to afirst rich air-fuel ratio that is rich of the stoichiometric air-fuelratio, until a time point when an output of the downstream-of-firstcatalyst air-fuel ratio sensor indicates an air-fuel ratio rich of thestoichiometric air-fuel ratio. After that, the upstream-of-firstcatalyst air-fuel ratio is controlled to a second rich air-fuel ratiothat is rich of the stoichiometric air-fuel ratio and that has a valuethat is determined in accordance with the oxidizing-reducing capabilityindex value, until a time point when an output of thedownstream-of-second catalyst air-fuel ratio sensor indicates anair-fuel ratio that is rich of the stoichiometric air-fuel ratio.

A maximum oxygen storage amount of the first catalyst is estimated bytaking into account the first rich air-fuel ratio to which theupstream-of-first catalyst air-fuel ratio was controlled. That is, themaximum oxygen storage amount of the first catalyst is estimated on thebasis of the amount of oxygen released (consumed) due to gas of thefirst rich air-fuel ratio. Furthermore, a maximum oxygen storage amountof the second catalyst is estimated by taking into account the value ofthe second rich air-fuel ratio to which the upstream-of-first catalystair-fuel ratio was controlled. That is, the maximum oxygen storageamount of the second catalyst is estimated on the basis of the amount ofoxygen released (consumed) due to gas of the second rich air-fuel ratio.

It is determined whether at least one of the first catalyst, the secondcatalyst and a catalyst device that includes the first catalyst and thesecond catalyst has degraded based on at least one of the estimatedmaximum oxygen storage of the first catalyst and the estimated maximumoxygen storage of the second catalyst.

In this catalyst degradation determining method, it is preferable toadopt a construction in which an arbitrary one or at least one of thefollowing determining operations is performed: determination as towhether the first catalyst has degraded on the basis of the estimatedmaximum oxygen storage amount of the first catalyst; determination as towhether the second catalyst has degraded on the basis of the estimatedmaximum oxygen storage amount of the second catalyst; determination asto whether the first catalyst has degraded on the basis of the estimatedmaximum oxygen storage amount of the first catalyst, as well asdetermination as to whether the second catalyst has degraded on thebasis of the estimated maximum oxygen storage amount of the secondcatalyst; and determination as to whether the catalyst device thatincludes the first catalyst and the second catalyst has degraded on thebasis of the estimated maximum oxygen storage amount of the firstcatalyst and the estimated maximum oxygen storage amount of the secondcatalyst.

Therefore, the time point at which the oxygen stored in the firstcatalyst is completely consumed can be reliably detected on the basis ofa change in the output of the downstream-of-first catalyst air-fuelratio sensor, so that the maximum oxygen storage amount of the firstcatalyst can be estimated with good precision. Furthermore, the timepoint at which the oxygen stored in the second catalyst is completelyconsumed can be reliably detected on the basis of a change in the outputof the downstream-of-second catalyst air-fuel ratio sensor, so that themaximum oxygen storage amount of the second catalyst can be estimatedwith good precision.

Furthermore, the second rich air-fuel ratio can be changed in accordancewith the degree of the oxidizing-reducing capability of the firstcatalyst and/or the degree of the oxidizing-reducing capability of thesecond catalyst (e.g., the degree of degradation of the first catalystand/or the degree of degradation of the second catalyst) indicated bythe oxidizing-reducing capability index value. Also, as mentioned above,the window range of each of the first and second catalysts narrows withprogression of degradation of the corresponding one of the catalysts.Therefore, for example, the second rich air-fuel ratio can be kept at anair-fuel ratio that is within the window range of the catalyst deviceformed by the first and second catalysts and that is near the lowerlimit value of the window range, regardless of the degrees ofdegradation of the first and second catalysts.

As a result, the efficiency of removal of unburned components, such asCO, HC, etc., based on the oxidizing function of the catalyst deviceimmediately after the time point at which the output of thedownstream-of-second catalyst air-fuel ratio sensor indicates anair-fuel ratio that is rich of the stoichiometric air-fuel ratio is keptat or above a predetermined high efficiency value, and therefore theamount of CO and HC emitted immediately after the aforementioned timepoint can be minimized. Furthermore, since the aforementioned secondrich air-fuel ratio is set at an air-fuel ratio that is as remote fromthe stoichiometric air-fuel ratio as possible, the period forcalculating the maximum oxygen storage amount of the second catalyst(i.e., the maximum oxygen storage amount calculation period for thefirst and second catalysts) can be reduced, in comparison with a casewhere the second rich air-fuel ratio is pre-set at a rich air-fuel ratiothat is near the stoichiometric air-fuel ratio.

In accordance with an embodiment of still another aspect of theinvention, there is provided a catalyst degradation determining methodas described below. In a catalyst degradation determining method appliedto an emission control apparatus of an internal combustion engine thatincludes: a catalyst disposed in an exhaust passage of the internalcombustion engine; and a downstream-of-catalyst air-fuel ratio sensordisposed in the exhaust passage downstream of the catalyst, anoxidizing-reducing capability index value that changes in accordancewith a degree of an oxidizing-reducing capability of the catalyst isacquired. Furthermore, an upstream-of-catalyst air-fuel ratio occurringupstream of the catalyst is controlled to an air-fuel ratio that is richof a stoichiometric air-fuel ratio so that the catalyst completelyreleases oxygen stored in the catalyst.

Then, the upstream-of-catalyst air-fuel ratio is controlled to a leanair-fuel ratio that is lean of the stoichiometric air-fuel ratio andthat has a value that is determined in accordance with theoxidizing-reducing capability index value, until a time point when anoutput of the downstream-of-catalyst air-fuel ratio sensor indicates anair-fuel ratio that is lean of the stoichiometric air-fuel ratio. Amaximum oxygen storage amount of the catalyst is estimated by takinginto account the value of the lean air-fuel ratio to which theupstream-of-catalyst air-fuel ratio was controlled. That is, the maximumoxygen storage amount of the catalyst is estimated on the basis of theamount of oxygen present in the gas of the aforementioned lean air-fuelratio.

It is determined whether the catalyst has degraded based on theestimated maximum oxygen storage amount of the catalyst. Theaforementioned “oxidizing-reducing capability index value” is preferablya degradation index value that changes in accordance with the degree ofcatalyst degradation or a value that changes in accordance with thecatalyst temperature. It is also preferable that the oxidizing-reducingcapability index value be a value based on the maximum oxygen storageamount of the catalyst already estimated by the above-described method.However, these examples are not restrictive. For example, theoxidizing-reducing capability index value may be a ratio between thelength of a locus of the output of an air-fuel ratio sensor disposedupstream of the catalyst and the length of a locus of the output of anair-fuel ratio sensor disposed downstream of the catalyst (locus ratio).

Therefore, the value of the aforementioned lean air-fuel ratio can bechanged in accordance with the degree of the oxidizing-reducingcapability of the catalyst (e.g., the degree of degradation of thecatalyst) indicated by the oxidizing-reducing capability index value.Furthermore, as mentioned above, the window range of the catalystnarrows as degradation of the catalyst progresses. Therefore, forexample, the aforementioned lean air-fuel ratio can be kept at anair-fuel ratio that is within the window range of the catalyst and thatis near the upper limit value of the window range, regardless of thedegree of degradation of the catalyst.

As a result, the efficiency of removal of nitrogen oxides NOx based onthe reducing function of the catalyst immediately after the time pointat which the output of the downstream-of-catalyst air-fuel ratio sensorindicates an air-fuel ratio that is lean of the stoichiometric air-fuelratio is kept at or above a predetermined high efficiency value, andtherefore the amount of nitrogen oxides NOx emitted immediately afterthe aforementioned time point can be minimized. Furthermore, since theaforementioned lean air-fuel ratio is set at an air-fuel ratio that isas remote from the stoichiometric air-fuel ratio as possible, the periodfor calculating the maximum oxygen storage amount of the catalyst can bereduced, in comparison with a case where the lean air-fuel ratio ispre-set at a lean air-fuel ratio that is near the stoichiometricair-fuel ratio.

If a first catalyst and a second catalyst are disposed in series in anexhaust passage of an internal combustion engine, there is provided acatalyst degradation determining method in accordance with a fourthaspect of the invention. That is, in a catalyst degradation determiningmethod applied to an emission control apparatus of an internalcombustion engine that includes: a first catalyst disposed in an exhaustpassage of the internal combustion engine; a downstream-of-firstcatalyst air-fuel ratio sensor disposed in the exhaust passagedownstream of the first catalyst; a second catalyst disposed in theexhaust passage downstream of the downstream-of-first catalyst air-fuelratio sensor; and a downstream-of-second catalyst air-fuel ratio sensordisposed in the exhaust passage downstream of the second catalyst, anoxidizing-reducing capability index value that changes in accordancewith at least one of a degree of an oxidizing-reducing capability of thefirst catalyst and a degree of an oxidizing-reducing capability of thesecond catalyst is acquired. Furthermore, an upstream-of-first catalystair-fuel ratio occurring upstream of the first catalyst is controlled toan air-fuel ratio that is rich of a stoichiometric air-fuel ratio sothat the first catalyst completely releases oxygen stored in the firstcatalyst and the second catalyst completely releases oxygen stored inthe second catalyst.

Then, the upstream-of-first catalyst air-fuel ratio is controlled to afirst lean air-fuel ratio that is lean of the stoichiometric air-fuelratio, until a time point when an output of the downstream-of-firstcatalyst air-fuel ratio sensor indicates an air-fuel ratio that is leanof the stoichiometric air-fuel ratio. After that, the upstream-of-firstcatalyst air-fuel ratio is controlled to a second lean air-fuel ratiothat is lean of the stoichiometric air-fuel ratio and that has a valuethat is determined in accordance with the oxidizing-reducing capabilityindex value, until a time point when an output of thedownstream-of-second catalyst air-fuel ratio sensor indicates anair-fuel ratio that is lean of the stoichiometric air-fuel ratio.

A maximum oxygen storage amount of the first catalyst is estimated bytaking into account the first lean air-fuel ratio to which theupstream-of-first catalyst air-fuel ratio was controlled. That is, themaximum oxygen storage amount of the first catalyst is estimated on thebasis of the amount of oxygen present in the gas of the first leanair-fuel ratio. A maximum oxygen storage amount of the second catalystis estimated by taking into account the value of the second leanair-fuel ratio to which the upstream-of-first catalyst air-fuel ratiowas controlled. That is, the maximum oxygen storage amount of the secondcatalyst is estimated on the basis of the amount of oxygen present inthe gas of the second lean air-fuel ratio.

It is determined whether at least one of the first catalyst, the secondcatalyst and a catalyst device that includes the first catalyst and thesecond catalyst has degraded based on at least one of the estimatedmaximum oxygen storage of the first catalyst and the estimated maximumoxygen storage of the second catalyst.

In this catalyst degradation determining method, it is preferable toadopt a construction in which an arbitrary one or at least one of thefollowing determining operations is performed: determination as towhether the first catalyst has degraded on the basis of the estimatedmaximum oxygen storage amount of the first catalyst; determination as towhether the second catalyst has degraded on the basis of the estimatedmaximum oxygen storage amount of the second catalyst; determination asto whether the first catalyst has degraded on the basis of the estimatedmaximum oxygen storage amount of the first catalyst, as well asdetermination as to whether the second catalyst has degraded on thebasis of the estimated maximum oxygen storage amount of the secondcatalyst; and determination as to whether the catalyst device thatincludes the first catalyst and the second catalyst has degraded on thebasis of the estimated maximum oxygen storage amount of the firstcatalyst and the estimated maximum oxygen storage amount of the secondcatalyst.

The aforementioned “oxidizing-reducing capability index value” ispreferably a degradation index value that changes in accordance with thedegree of degradation of the first catalyst and/or the degree ofdegradation of the second catalyst or a value that changes in accordancewith the temperature of the first catalyst and/or the temperature of thesecond catalyst. It is also preferable that the oxidizing-reducingcapability index value be a value based on the maximum oxygen storageamount of the first catalyst already estimated by the above-describedmethod and/or the maximum oxygen storage amount of the second catalystalready estimated by the above-described method (e.g., a value obtainedby summing the maximum oxygen storage amount of the first catalyst andthe maximum oxygen storage amount of the second catalyst alreadyestimated by the above-described method). However, these examples arenot restrictive. The first lean air-fuel ratio may be, for example, anair-fuel ratio equal to the second lean air-fuel ratio.

Therefore, the time point at which the oxygen stored in the firstcatalyst reaches the maximum oxygen storage amount can be reliablydetected on the basis of a change in the output of thedownstream-of-first catalyst air-fuel ratio sensor, so that the maximumoxygen storage amount of the first catalyst can be estimated with goodprecision. Furthermore, the time point at which the oxygen stored in thesecond catalyst reaches the maximum oxygen storage amount can bereliably detected on the basis of a change in the output of thedownstream-of-second catalyst air-fuel ratio sensor, so that the maximumoxygen storage amount of the second catalyst can be estimated with goodprecision.

Furthermore, similarly to the catalyst degradation determining method inaccordance with the third aspect of the invention, the second leanair-fuel ratio can be changed in accordance with the degree of theoxidizing-reducing capability of the first catalyst and/or the degree ofthe oxidizing-reducing capability of the second catalyst (e.g., thedegree of degradation of the first catalyst and/or the degree ofdegradation of the second catalyst) indicated by the oxidizing-reducingcapability index value. Also, as mentioned above, the window range ofeach of the first and second catalysts narrows with progression ofdegradation of the corresponding one of the catalysts. Therefore, forexample, the second lean air-fuel ratio can be kept at an air-fuel ratiothat is within the window range of the catalyst device formed by thefirst and second catalysts and that is near the upper limit value of thewindow range, regardless of the degrees of degradation of the first andsecond catalysts.

As a result, the efficiency of removal of nitrogen oxides NOx based onthe reducing function of the catalyst device immediately after the timepoint at which the output of the downstream-of-second catalyst air-fuelratio sensor indicates an air-fuel ratio that is lean of thestoichiometric air-fuel ratio is kept at or above a predetermined highefficiency value, and therefore the amount of nitrogen oxides NOxemitted immediately after the aforementioned time point can beminimized. Furthermore, since the aforementioned second lean air-fuelratio is set at an air-fuel ratio that is as remote from thestoichiometric air-fuel ratio as possible, the period for calculatingthe maximum oxygen storage amount of the second catalyst (i.e., themaximum oxygen storage amount calculation period for the first andsecond catalysts) can be reduced, in comparison with a case where thesecond lean air-fuel ratio is pre-set at a lean air-fuel ratio that isnear the stoichiometric air-fuel ratio.

In the above-described catalyst degradation determining methods, if theemission control apparatus of the internal combustion engine to whichthe catalyst degradation determining method is applied includes anupstream-of-catalyst air-fuel ratio sensor disposed in the exhaustpassage upstream of the catalyst or upstream of the first catalyst, andan upstream-of-catalyst air-fuel ratio sensor abnormality detector fordetecting an abnormality of the upstream-of-catalyst air-fuel ratiosensor, it is preferable to adopt a construction in which the maximumoxygen storage amount of the catalyst or the maximum oxygen storageamounts of the first catalyst and the second catalyst are estimatedbased on an output of the upstream-of-catalyst air-fuel ratio sensor,and in which a determination that the catalyst has degraded isprohibited in a case where the catalyst is in a state in which it is tobe determined that the catalyst has degraded based on the estimatedmaximum oxygen storage amount, and where an abnormality of theupstream-of-catalyst air-fuel ratio sensor has been detected, and inwhich it is determined that the catalyst has not degraded regardless ofwhether an abnormality of the upstream-of-catalyst air-fuel ratio sensorhas been detected, in a case where the catalyst is in a state in whichit is to be determined that the catalyst has not degraded based on theestimated maximum oxygen storage amount.

In the case where the maximum oxygen storage amount of the catalyst orthe maximum oxygen storage amounts of the first catalyst and the secondcatalyst are estimated on the basis of the output of theupstream-of-catalyst air-fuel ratio sensor, that is, in the case wherethe maximum oxygen storage amount of the catalyst or the maximum oxygenstorage amounts of the first catalyst and the second catalyst areestimated by calculating the amount of oxygen released due to gas of arich air-fuel ratio or the amount of oxygen present in gas of a leanair-fuel ratio on the basis of the amount of deviation of the presentvalue of output of the upstream-of-catalyst air-fuel ratio sensor fromthe value of output of the upstream-of-catalyst air-fuel ratio sensorthat occurs when the air-fuel ratio of the gas detected by theupstream-of-catalyst air-fuel ratio sensor is the stoichiometricair-fuel ratio (hereinafter, referred to as “stoichiometric air-fuelratio-time output value”), it becomes impossible to acquire an accuratemaximum oxygen storage amount if the upstream-of-catalyst air-fuel ratiosensor has an abnormality. Therefore, if an abnormality of theupstream-of-catalyst air-fuel ratio sensor is detected by theupstream-of-catalyst air-fuel ratio sensor abnormality detector,determination regarding catalyst degradation based on the estimatedmaximum oxygen storage amount as described above may possibly fail toprovide a precise result of determination regarding catalystdegradation.

However, in general, as degradation of an air-fuel ratio sensor (agenerally-termed concentration cell type oxygen sensor or agenerally-termed limiting current type oxygen sensor) progresses, theamount of change in the output of the air-fuel ratio sensor with respectto an actual amount of change in the air-fuel ratio of gas subjected todetection by the air-fuel ratio sensor tends to decrease. That is, theamount of deviation of the value of output of the air-fuel ratio sensorfrom the stoichiometric air-fuel ratio-time output value with respect tothe amount of deviation of the actual air-fuel ratio of gas subjected todetection by the air-fuel ratio sensor from the stoichiometric air-fuelratio tends to decrease as degradation of the sensor progresses. Inother words, as degradation of the upstream-of-catalyst air-fuel ratiosensor progresses (if the upstream-of-catalyst air-fuel ratio sensor hasan abnormality), the maximum oxygen storage amount of the catalystestimated on the basis of the output of the upstream-of-catalystair-fuel ratio sensor becomes less than the actual maximum oxygenstorage amount of the catalyst.

Therefore, in the case where it is determined whether a catalyst hasdegraded on the basis of whether the value indicating the maximum oxygenstorage amount of the catalyst is less than or equal to a predetermineddegradation criterion value, if the value indicating the maximum oxygenstorage amount of the catalyst estimated on the basis of the output ofthe upstream-of-catalyst air-fuel ratio sensor is greater than thedegradation criterion value while an abnormality of theupstream-of-catalyst air-fuel ratio sensor has been detected, it iscertain that the value indicating the actual maximum oxygen storageamount of the catalyst is sufficiently greater than the degradationcriterion value, and therefore a determination that the catalyst has notdegraded can be correctly and precisely made despite the sensorabnormality. However, if the value indicating the maximum oxygen storageamount of the catalyst estimated on the basis of the output of theupstream-of-catalyst air-fuel ratio sensor is less than or equal to thedegradation criterion value while an abnormality of theupstream-of-catalyst air-fuel ratio sensor has been detected, themagnitude relationship between the value indicating the actual maximumoxygen storage amount of the catalyst and the degradation criterionvalue is uncertain, and therefore a precise result of determinationregarding degradation of the catalyst cannot be obtained.

According to the above-described construction, even if an abnormality ofthe upstream-of-catalyst air-fuel ratio sensor has been detected as aresult of progression of degradation of the upstream-of-catalystair-fuel ratio sensor, a determination that the catalyst has notdegraded can be made. As long as a determination that the catalyst hasnot degraded is made, there is no need to replace theupstream-of-catalyst air-fuel ratio sensor. Thus, the time to replacethe upstream-of-catalyst air-fuel ratio sensor can be delayed.Furthermore, since a determination that the catalyst has degraded isavoided if an abnormality of the upstream-of-catalyst air-fuel ratiosensor has been detected, a false determination regarding degradation ofthe catalyst can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned and other objects, features, advantages, technicaland industrial significance of this invention will be better understoodby reading the following detailed description of preferred exemplaryembodiments of the invention, when considered in connection with theaccompanying drawings, in which:

FIG. 1 is a schematic illustration of an internal combustion engineequipped with an emission control apparatus (catalyst degradationdetermining apparatus) for carrying out a catalyst degradationdetermining method in accordance with an embodiment of the invention;

FIG. 2 is a map indicating a relationship between the output voltage ofan air flow meter shown in FIG. 1 and the measured amount of intake air;

FIG. 3 is a map indicating a relationship between the output voltage ofan upstream-most air-fuel ratio sensor shown in FIG. 1 and the air-fuelratio;

FIG. 4 is a map indicating a relationship between the output voltages ofa downstream-of-first catalyst air-fuel ratio sensor and adownstream-of-second catalyst air-fuel ratio sensor shown in FIG. 1 andthe air-fuel ratio;

FIG. 5 is a time chart indicating changes in the controlledupstream-of-first catalyst air-fuel ratio, the outputs of the air-fuelratio sensors and the amounts of oxygen stored in catalysts during theexecution of determination regarding catalyst degradation by thecatalyst degradation determining apparatus;

FIG. 6 is a flowchart illustrating a routine for calculating the amountof fuel injection executed by a CPU shown in FIG. 1;

FIG. 7 is a flowchart illustrating a routine for calculating an air-fuelratio feedback correction amount executed by the CPU shown in FIG. 1;

FIG. 8 is a flowchart illustrating a routine for calculating asubsidiary feedback control amount executed by the CPU shown in FIG. 1;

FIG. 9 is a flowchart illustrating a routine for determining whether tostart the determination regarding catalyst degradation executed by theCPU shown in FIG. 1;

FIG. 10 is a flowchart illustrating a routine of a first mode executedby the CPU shown in FIG. 1;

FIG. 11 is a flowchart illustrating a routine of a second mode executedby the CPU shown in FIG. 1;

FIG. 12 is a flowchart illustrating a routine of a third mode executedby the CPU shown in FIG. 1;

FIG. 13 is a flowchart illustrating a routine of a fourth mode executedby the CPU shown in FIG. 1;

FIG. 14 is a flowchart illustrating a routine of a fifth mode executedby the CPU shown in FIG. 1;

FIG. 15 is a flowchart illustrating a routine of a sixth mode executedby the CPU shown in FIG. 1;

FIG. 16 is a flowchart illustrating a routine for calculating an oxygenstorage amount executed by the CPU shown in FIG. 1; and

FIG. 17 a and FIG. 17 b are flowcharts illustrating a routine fordetermining whether the upstream-most air-fuel ratio sensor has anabnormality and determining whether a catalyst has degraded executed bythe CPU shown in FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following description and the accompanying drawings, the presentinvention will be described in more detail with reference to exemplaryembodiments.

FIG. 1 is a schematic illustration of the construction of a system inwhich an emission control apparatus (catalyst degradation determiningapparatus) for carrying out a catalyst degradation determining method inaccordance with an embodiment of the invention is applied to a sparkignition type multi-cylinder (e.g., four-cylinder) internal combustionengine 10.

The internal combustion engine 10 includes a cylinder block section 20that includes a cylinder block lower case, an oil pan, etc., a cylinderhead section 30 fixed to the cylinder block section 20, an intake system40 for supplying gasoline mixture to the cylinder block section 20, andan exhaust system 50 for releasing exhaust gas from the cylinder blocksection 20 to the outside.

The cylinder block section 20 includes cylinders 21, pistons 22,connecting rods 23, and a crankshaft 24. The pistons 22 reciprocatewithin the cylinders 21. The reciprocating movements of the pistons 22are transferred to the crankshaft 24 via the connecting rods 23, therebyrotating the crankshaft 24. The cylinders 21, heads of the pistons 22and the cylinder head section 30 define combustion chambers 25.

The cylinder head section 30 includes intake ports 31 connected incommunication to the combustion chambers 25, intake valves 32 that openand close the intake ports 31, a variable intake timing device 33 thatincludes an intake camshaft for driving the intake valves 32, and thatcontinuously changes the phase angle of the intake camshaft, and anactuator 33 a of the variable intake timing device 33. The cylinder headsection 30 also includes exhaust ports 34 connected in communication tothe combustion chambers 25, exhaust valves 35 that open and close theexhaust ports 34, an exhaust camshaft 36 for driving the exhaust valves35, ignition plugs 37, an igniter 38 that includes an ignition coil forproducing a high voltage to be supplied to the ignition plugs 37, andinjectors (fuel injection means) 39 that inject fuel into the intakeports 31.

The intake system 40 includes an intake pipe 41 that is connected incommunication to the intake ports 31 and that includes an intakemanifold that forms an intake passage together with the intake ports 31,an air filter 42 provided at an end portion of the intake pipe 41, athrottle valve 43 provided within the intake pipe 41 for varying thesectional area of the opening of the intake passage, a throttle valveactuator 43 a formed by a DC motor which forms throttle valve drivemeans, a swirl control valve (hereinafter, referred to as “SCV”) 44, andan SCV actuator 44 a formed by a DC motor.

The exhaust system 50 includes an exhaust manifold 51 connected incommunication to the exhaust ports 34, an exhaust pipe 52 connected incommunication to the exhaust manifold 51, an upstream-side firstcatalyst (also referred to as “upstream-side three-way catalyst” or“start converter”) 53 disposed in the exhaust pipe 52 (between exhaustpipe portions), a second catalyst (also referred to as “downstream-sidethree-way catalyst” or as “under-floor converter” due to the mountingbelow a floor of a vehicle) 54 disposed in the exhaust pipe 52 (betweenexhaust pipe portions) downstream of the first catalyst 53. The exhaustports 34, the exhaust manifold 51 and the exhaust pipe 52 form anexhaust passage.

This system includes a hot wire air flow meter 61, a throttle positionsensor 62, a cam position sensor 63, a crank position sensor 64, a watertemperature sensor 65, an air-fuel ratio sensor 66 disposed in theexhaust passage upstream of the first catalyst 53 (hereinafter, referredto as “upstream-most air-fuel ratio sensor 66”), an air-fuel ratiosensor 67 disposed in the exhaust passage downstream of the firstcatalyst 53 and upstream of the second catalyst 54 (hereinafter,referred to as “downstream-of-first catalyst air-fuel ratio sensor 67”),an air-fuel ratio sensor 68 disposed downstream of the second catalyst54 (hereinafter, referred to as “downstream-of-second catalyst air-fuelratio sensor 68”), and an accelerator operation amount sensor 69.

The hot wire air flow meter 61 is designed to output a voltage Vgcorresponding to the mass flow of intake air that flows in the intakepipe 41. A relationship between the output Vg of the air flow meter 61and the measured intake air amount (flow) AFM is indicated in FIG. 2.The throttle position sensor 62 is designed to detect the degree ofopening of the throttle valve 43 and output a signal that indicates thedegree of throttle valve opening TA. The cam position sensor 63 isdesigned to generate a signal (G2 signal) having a pulse at everyrotational angle of 90° of the intake camshaft (that is, everyrotational angle of 180° of the crankshaft 24). The crank positionsensor 64 is designed to output a signal that has a narrow pulse atevery 10° rotation of the crankshaft 24 and has a wide pulse at every360° rotation of the crankshaft 24. This signal indicates the enginerotation speed NE. The water temperature sensor 65 is designed to detectthe temperature of cooling water of the internal combustion engine 10and output a signal indicating the cooling water temperature THW.

The upstream-most air-fuel ratio sensor 66 is a generally-termedlimiting current type oxygen sensor, and is designed to output anelectric current corresponding to the air-fuel ratio A/F, and output avoltage vabyfs corresponding to the current as indicated by a solid linein FIG. 3. As is apparent from FIG. 3, the upstream-most air-fuel ratiosensor 66 allows high-precision detection of the air-fuel ratio A/F overa wide range.

The downstream-of-first catalyst air-fuel ratio sensor 67 and thedownstream-of-second catalyst air-fuel ratio sensor 68 aregenerally-termed concentration cell type oxygen sensors, and aredesigned to output voltages Voxs1, Voxs2, respectively, that sharplychange at the stoichiometric air-fuel ratio. More specifically, as shownin FIG. 4, the downstream-of-first catalyst air-fuel ratio sensor 67 andthe downstream-of-second catalyst air-fuel ratio sensor 68 output about0.1 (V) when the air-fuel ratio is lean of the stoichiometric air-fuelratio, and output about 0.9 (V) when the air-fuel ratio is rich of thestoichiometric air-fuel ratio, and output about 0.5 (V) when theair-fuel ratio is equal to the stoichiometric air-fuel ratio. Theaccelerator operation amount sensor 69 is designed to detect the amountof operation of an accelerator pedal 81 operated by a driver, and tooutput a signal indicating the operation amount Accp of the acceleratorpedal 81.

This system further includes an electric control unit 70. The electriccontrol unit 70 is a microcomputer that includes a CPU 71, a ROM 72 inwhich routines (programs) executed by the CPU 71, tables (look-uptables, maps), constants, etc., are pre-stored, a RAM 73 into which theCPU 71 temporarily stores data when necessary, a backup RAM 74 thatstores data when a power supply is on, and that retains the stored datawhile the power supply is off, an interface 75 that includes an ADconverter, etc., which are interconnected by a bus. The interface 75 isconnected to the sensors 61 to 69, and supplies signals from the sensors61 to 69 to the CPU 71, and sends drive signals to the actuator 33 a ofthe variable intake timing device 33, the igniter 38, the injectors 39,the throttle valve actuator 43 a, and the SCV actuator 44 a.

(Principle of Determination of Catalyst Degradation)

Three-way catalysts, such as the first and second catalysts 53, 54, havethe function of oxidizing unburned components (HC, CO) (oxidizingfunction) and the function of reducing nitrogen oxides (NOx) (reducingfunction), and are able to substantially remove harmful components,including the unburned components and nitrogen oxides, due to theoxidizing and reducing functions. The efficiency of removal of harmfulcomponents by the oxidizing and reducing functions of the three-waycatalyst increases as the internal combustion engine air-fuel ratioapproaches the vicinity of the stoichiometric air-fuel ratio. Therefore,if the engine air-fuel ratio is kept within a window range that containsthe stoichiometric air-fuel ratio, the removal efficiency can be kept ator above a predetermined high value.

Furthermore, the three-way catalyst also has the oxygen storing andreleasing functions. Due to the oxygen storing and releasing functions,the catalyst is able to substantially remove HC, CO and NOx even if theair-fuel ratio deviates from the stoichiometric air-fuel ratio to acertain extent. That is, if the engine air-fuel ratio becomes lean ofthe stoichiometric air-fuel ratio and the gas flowing into the three-waycatalyst comes to contain a large amount of NOx, the three-way catalystdetaches oxygen molecules from NOx and therefore reduces NOx, that is,removes NOx, and stores the oxygen, due to its oxygen storing function.If the engine air-fuel ratio becomes rich of the stoichiometric air-fuelratio and the gas flowing into the three-way catalyst comes to containlarge amounts of HC and CO, the catalyst oxidizes and therefore removesHC and CO by giving them stored oxygen due to the oxygen releasingfunction of the catalyst.

Therefore, in order to efficiently remove large amounts of HC and COcontinuously flowing into the three-way catalyst, the catalyst needs tohave a large amount of stored oxygen. Conversely, in order toefficiently remove a large amount of NOx continuously flowing into thethree-way catalyst, the catalyst needs to be in a state where thecatalyst is able to store a sufficiently large amount of oxygen.

As is apparent from the above discussion, the emissions controlcapability of the three-way catalyst is dependent on the maximum oxygenstorage amount, that is, the maximum amount of oxygen that the three-waycatalyst is able to store (absorb). However, the three-way catalystdegrades due to the poisoning caused by lead, sulfur and the likecontained in fuel, or heat given to the catalyst. Therefore, the maximumoxygen storage amount of the catalyst gradually decreases. That is, ifthe maximum oxygen storage amounts of the first and second catalysts 53,54 are estimated, it becomes possible to determine whether the catalystshave degraded, separately for the individual catalysts. Furthermore, onthe basis of the combination of results of the determination, it ispossible to determine degradation of a catalyst device that is formed bythe first and second catalysts 53, 54.

Therefore, the catalyst degradation determining apparatus of theembodiment estimates maximum oxygen storage amounts of the first andsecond catalysts 53, 54 by forcibly changing the air-fuel ratio of gasupstream of the first catalyst 53 from the fuel-rich side to thefuel-lean side of the stoichiometric air-fuel ratio (or in the oppositedirection), as described below. In this case, harmful components arelikely to be emitted immediately after the switch of theupstream-of-first catalyst air-fuel ratio from a rich air-fuel ratio toa lean air-fuel ratio (or in the opposite direction) as mentioned above.Therefore, at this time point, it is necessary to lessen the emission ofharmful components. Furthermore, it is preferable that the period neededto estimate the oxygen storage amount be short.

Considering the aforementioned requirements, the apparatus of theembodiment minimizes the emission of harmful components, and estimatesthe maximum oxygen storage amounts of the first and second catalysts 53,54 within a relatively short period of time as indicated in time chartsof FIG. 5. First, at a time point t1, as indicated in (A) of FIG. 5, theapparatus controls the air-fuel ratio of gas upstream of the firstcatalyst 53 (which in reality is the air-fuel ratio of a mixture takeninto the internal combustion engine, and will sometimes be referred toas “upstream-of-first catalyst air-fuel ratio” in the followingdescription) to a first lean air-fuel ratio that is lean of thestoichiometric air-fuel ratio. The first lean air-fuel ratio is set atan air-fuel ratio that is slightly above the upper limit value of thewindow range at the time of brand new condition of a catalyst devicethat is a unit of the first and second catalysts 53, 54.

Therefore, since a gas having a lean air-fuel ratio flows into the firstcatalyst 53, the amount of oxygen stored in the first catalyst 53gradually increases, and reaches the maximum oxygen storage amountCSCmax at a time point t2 as indicated in (C) of FIG. 5. As a result, atthe time point t2, a gas containing oxygen (lean air-fuel ratio gas)starts to flow out of the first catalyst 53, and the output Voxs1 of thedownstream-of-first catalyst air-fuel ratio sensor 67 changes from arich state-indicating value to a lean state-indicating value asindicated in (B) of FIG. 5. The operation performed between the timepoints t1 and t2 is termed operation in the first mode (Mode=1). As theoutput Voxs1 of the downstream-of-first catalyst air-fuel ratio sensor67 changes from the rich state-indicating value to the leanstate-indicating value at the time point t2, the apparatus stores thevalue of output vabyfs of the upstream-most air-fuel ratio sensor 66occurring at the end of the first mode (Mode=1) (i.e., during the statewhere the upstream-of-first catalyst air-fuel ratio is controlled to thefirst lean air-fuel ratio) into a variable VabyfsL, and then controlsthe upstream-of-first catalyst air-fuel ratio to a second lean air-fuelratio that is lean of the stoichiometric air-fuel ratio and that is richof the first lean air-fuel ratio (see (A) of FIG. 5).

The second lean air-fuel ratio is changed in accordance with the valueof a maximum oxygen storage amount CRaxall of the entire catalystdevice, that is, the sum of the value of maximum oxygen storage amountCUFmax of the second catalyst 54 and the value of the maximum oxygenstorage amount CSCmax of the first catalyst 53 estimated by theapparatus during the previous catalyst degradation determiningoperation. The second lean air-fuel ratio is set (relatively great) soas to approach an air-fuel ratio near the upper limit value of thewindow range provided at the time of brand new condition of the catalystdevice as the value of maximum oxygen storage amount Cmaxall increases,and is set (relatively small) so as to approach the stoichiometricair-fuel ratio as the value of maximum oxygen storage amount Cmaxalldecreases (as the degradation of the first catalyst 53 and the secondcatalyst 54 (catalyst device) progresses). As a result, the second leanair-fuel ratio is set so as to become an air-fuel ratio near the upperlimit value of the window range provided at a given time, which windowrange narrows in accordance with the degree of degradation of thecatalyst device.

In this state, therefore, a gas having a lean air-fuel ratio flows intothe first catalyst 53, and the oxygen storage amount of the firstcatalyst 53 is maximum and no more oxygen can be stored into the firstcatalyst 53. Therefore, an oxygen-containing gas continues flowing outof the first catalyst 53. As a result, from the time point t2 on, theoxygen storage amount of the second catalyst 54 gradually increases, andreaches the maximum oxygen storage amount CUFmax at a time point t3, asindicated in (E) of FIG. 5. Therefore, at the time point t3, anoxygen-containing gas starts to flow out of the second catalyst 54, andthe output Voxs2 of the downstream-of-second catalyst air-fuel ratiosensor 68 changes from a rich state-indicating value to a leanstate-indicating value as indicated in (D) of FIG. 5. The operationperformed between the time points t2 and t3 is termed operation in thesecond mode (Mode=2).

As described above, during the first and second modes (Mode=1, Mode=2),the air-fuel ratio of gas upstream of the first catalyst 53 iscontrolled to the lean side of the stoichiometric air-fuel ratio so thatthe first catalyst 53 stores oxygen therein up to the limit of theoxygen storage capability thereof, and the second catalyst 54 storesoxygen therein up to the limit of the oxygen storage capability thereof,respectively.

As the output Voxs2 of the downstream-of-second catalyst air-fuel ratiosensor 68 changes from the rich state-indicating value to the leanstate-indicating value at the time point t3, the apparatus controls theupstream-of-first catalyst air-fuel ratio to a first rich air-fuel ratiothat is rich of the stoichiometric air-fuel ratio. The first richair-fuel ratio is set at an air-fuel ratio that is slightly below thelower limit value of the window range provided at the time of brand newcondition of the catalyst device that is a unit of the first and secondcatalysts 53, 54.

Therefore, a rich air-fuel ratio gas flows into the first catalyst 53,so that oxygen stored in the first catalyst 53 is consumed to oxidizethe unburned components, that is, HC and CO, which enter the firstcatalyst 53. Hence, the oxygen storage amount of the first catalyst 53decreases from the maximum oxygen storage amount CSCmax. Then, at a timepoint t4, the oxygen storage amount of the first catalyst 53 reaches“0”. Therefore, a rich air-fuel ratio gas starts to flow out of thefirst catalyst 53, and the output Voxs1 of the downstream-of-firstcatalyst air-fuel ratio sensor 67 changes from a lean state-indicatingvalue to a rich state-indicating value. The operation performed betweenthe time points t3 and t4 is termed operation in the third mode(Mode=3).

During the period between the time points t3 and t4, the apparatusestimates the maximum oxygen storage amount CSCmax of the first catalyst53 as a maximum oxygen storage amount CSCmax3 as described below. Thatis, during the period from the time point t3 when the upstream-of-firstcatalyst air-fuel ratio is set at the first rich air-fuel ratio to thetime point t4 when the output Voxs1 of the downstream-of-first catalystair-fuel ratio sensor 67 changes to the rich state-indicating value, theapparatus calculates amounts of change ΔO2 in the oxygen storage amountand accumulates the amounts of change ΔO2 as in mathematical expressions1 and 2, thereby calculating the accumulated value at the time point t4as a maximum oxygen storage amount CSCmax3.ΔO2=0.23×mfr×(stoich−abyfs)  [Expression 1]CSCmax3=τΔO2(time section t=t 3 to t 4) [Expression 2]

In mathematical expression 1, the value “0.23” is the weight proportionof oxygen contained in the atmosphere. mfr is the total of fuelinjection amount Fi within a predetermined time (calculation cycletsample), and stoich is the stoichiometric air-fuel ratio (e.g., 14.7).abyfs is the air-fuel ratio A/F detected by the upstream-most air-fuelratio sensor 66 in the predetermined time tsample. abyfs may be the meanvalue of the air-fuel ratios A/F detected by the upstream-most air-fuelratio sensor 66 within the predetermined time tsample.

As indicated in mathematical expression 1, the multiplication of thetotal mfr of the fuel injection amount Fi within the predetermined timetsample by the shift (stoich−abvfs) of the detected air-fuel ratio A/Ffrom the stoichiometric air-fuel ratio provides the shortfall of air inthe predetermined time tsample, and the multiplication of the shortfallof air by the weight proportion of oxygen provides the amount of changeΔO2 in the oxygen storage amount (i.e, amount of consumption of storedoxygen) in the predetermined time tsample. Then, as indicated inmathematical expression 2, the accumulation of the amounts of change ΔO2in the oxygen storage amount over the time of t3 to t4 provides anestimate of the amount of oxygen consumed during the period starting atthe state where the amount of oxygen stored in the first catalyst 53 ismaximum and ending at the complete consumption of the oxygen, that is,the maximum oxygen storage amount CSCmax3. Thus, in this embodiment, themaximum oxygen storage amount CSCmax3 is estimated on the basis of theoutput of the upstream-most air-fuel ratio sensor 66 (by utilizing thefact that the upstream-of-first catalyst air-fuel ratio is controlled tothe first rich air-fuel ratio).

As the output of the downstream-of-first catalyst air-fuel ratio sensor67 changes from the lean state-indicating value to the richstate-indicating value at the time point t4, the apparatus stores thevalue of output vabyfs of the upstream-most air-fuel ratio sensor 66occurring at the end of the third mode (Mode=3) (i.e., during the statewhere the upstream-of-first catalyst air-fuel ratio is controlled to thefirst rich air-fuel ratio) into a variable VabyfsR, and then controlsthe upstream-of-first catalyst air-fuel ratio to a second rich air-fuelratio that is rich of the stoichiometric air-fuel ratio and that is leanof the first rich air-fuel ratio.

The second rich air-fuel ratio is changed in accordance with the valueof the maximum oxygen storage amount CRaxall of the entire catalystdevice estimated by the apparatus during the previous catalystdegradation determining operation. The second rich air-fuel ratio is set(relatively small) so as to approach an air-fuel ratio near the lowerlimit value of the window range provided at the time of brand newcondition of the catalyst device as the value of maximum oxygen storageamount Cmaxall increases, and is set (relatively large) so as toapproach the stoichiometric air-fuel ratio as the value of maximumoxygen storage amount Cmaxall decreases (as the degradation of the firstcatalyst 53 and the second catalyst 54 (catalyst device) progresses). Asa result, the second rich air-fuel ratio is set so as to become anair-fuel ratio near the lower limit value of the window range providedat a given time, which window range narrows in accordance with thedegree of degradation of the catalyst device.

At this time, the oxygen storage amount of the first catalyst 53 is “0”,and therefore, a rich air-fuel ratio gas flows into the second catalyst54. As a result, oxygen stored in the second catalyst 54 is consumed tooxidize the unburned components, that is, HC and CO, which enter thesecond catalyst 54. Therefore, the oxygen storage amount of the secondcatalyst 54 decreases from the maximum oxygen storage amount CUFmax.

Then, at a time point t5, the amount of oxygen stored in the secondcatalyst 54 reaches “0”. Therefore, a rich air-fuel ratio gas starts toflow out of the second catalyst 54, and the output Voxs2 of thedownstream-of-second catalyst air-fuel ratio sensor 68 changes from alean state-indicating value to a rich state-indicating value. That is,during the period between the time points t3 and t5, the air-fuel ratioof gas upstream of the first catalyst 53 is controlled to the rich sideof the stoichiometric air-fuel ratio so that the oxygen stored in thefirst catalyst 53 is completely released during the period between thetime points t3 and t4, and the oxygen stored in the second catalyst 54is completely released during the period between the time points t4 andt5. The operation performed between the time points t4 and t5 is termedoperation in the fourth mode (Mode=4).

During the period between the time points t4 and t5, the apparatuscalculates and estimates the maximum oxygen storage amount CUFmax of thesecond catalyst 54 as a maximum oxygen storage amount CUFmax4 throughcalculations as in mathematical expressions 3 and 4, which are similarto those used in the above-described calculation of the maximum oxygenstorage amount CSCmax3. Thus, in this embodiment, the maximum oxygenstorage amount CUFmax4 is estimated on the basis of the output of theupstream-most air-fuel ratio sensor 66 (by utilizing the fact that theupstream-of-first catalyst air-fuel ratio is controlled to the secondrich air-fuel ratio).ΔO2=0.23×mfr×(stoich−abyfs)  [Expression 3]CUFmax4=τΔO2(time section=t 4 to t 5)  [Expression 4]

As the output Voxs2 of the downstream-of-second catalyst air-fuel ratiosensor 68 changes from the lean state-indicating value to the richstate-indicating value at the time point t5, the apparatus controls theupstream-of-first catalyst air-fuel ratio to the aforementioned firstlean air-fuel ratio that is lean of the stoichiometric air-fuel ratio.Therefore, a lean air-fuel ratio gas flows into the first catalyst 53.At the time point t5, the oxygen storage amount of the first catalyst 53is “0”. Therefore, the amount of oxygen stored in the first catalyst 53continuously increases from “0” from the time point t5 on, and reachesthe maximum oxygen storage amount CSCmax at a time point t6. As aresult, at the time point t6, an oxygen-containing gas starts to flowout of the first catalyst 53, and the output Voxs1 of thedownstream-of-first catalyst air-fuel ratio sensor 67 changes from arich state-indicating value to a lean state-indicating value. Theoperation performed between the time points t5 and t6 is termedoperation in the fifth mode (Mode=5).

During the period between the time points t5 and t6, too, the apparatusestimates the maximum oxygen storage amount CSCmax of the first catalyst53 as a maximum oxygen storage amount CSCmax5 as described below. Thatis, the time point t6 when the output Voxs1 of the downstream-of-firstcatalyst air-fuel ratio sensor 67 changes to the lean air-fuelratio-indicating value is a time point when the oxygen storage amount ofthe first catalyst 53 reaches the maximum oxygen storage amount CSCmax.Therefore, during the period between the time points t5 and t6, theapparatus calculates and accumulates amounts of change ΔO2 in the oxygenstorage amount as in mathematical expressions 5 and 6, therebycalculating and estimating the accumulated value at the time point t6 asthe maximum oxygen storage amount CSCmax5.ΔO2=0.23×mfr×(abyfs−stoich)  [Expression 5]CSCmax5=ΣΔO2(time section t=t 5 to t 6)  [Expression 6]

As indicated in mathematical expression 5, the multiplication of thetotal mfr of the fuel injection amount within the predetermined timetsample by the shift (abyfs−stoich) of the air-fuel ratio A/F from thestoichiometric air-fuel ratio provides the excess amount of air in thepredetermined time tsample, and the multiplication of the excess amountof air by the weight proportion of oxygen provides the amount of changeΔO2 in the oxygen storage amount (i.e, amount of oxygen stored) in thepredetermined time tsample. Then, as indicated in mathematicalexpression 6, the accumulation of the amounts of change ΔO2 in theoxygen storage amount over the time of t5 to t6 provides an estimate ofthe amount of oxygen stored during the period starting at the statewhere the amount of oxygen stored in the first catalyst 53 is “0” andending at the maximum storage of oxygen, that is, the maximum oxygenstorage amount CSCmax5. Thus, in this embodiment, the maximum oxygenstorage amount CSCmax5 is estimated on the basis of the output of theupstream-most air-fuel ratio sensor 66 (by utilizing the fact that theupstream-of-first catalyst air-fuel ratio is controlled to the firstlean air-fuel ratio).

As the output Voxs1 of the downstream-of-first catalyst air-fuel ratiosensor 67 changes from the rich state-indicating value to the leanstate-indicating value at the time point t6, the apparatus controls theupstream-of-first catalyst air-fuel ratio to the aforementioned secondlean air-fuel ratio that is lean of the stoichiometric air-fuel ratioand that is rich of the first lean air-fuel ratio. In this case, theoxygen storage amount of the first catalyst 53 has reached the maximumoxygen storage amount CSCmax. Therefore, a lean air-fuel ratio gas flowsout of the first catalyst 53, and flows into the second catalyst 54. Atthe time point t6, the oxygen storage amount of the second catalyst 54is “0”. Therefore, the amount of oxygen stored in the second catalyst 54continuously increases from “0” from the time point t6 on, and reachesthe maximum oxygen storage amount CUFmax at a time point t7. As aresult, at the time point t7, an oxygen-containing gas starts to flowout of the second catalyst 54, and the output Voxs2 of thedownstream-of-second catalyst air-fuel ratio sensor 68 changes from arich state-indicating value to a lean state-indicating value. Theoperation performed between the time points t6 and t7 is termedoperation in the sixth mode (Mode=6).

During the period between the time points t6 and t7, too, the apparatusestimates the maximum oxygen storage amount CUFmax of the secondcatalyst 54 as a maximum oxygen storage amount CUFmax6 as describedbelow. That is, the apparatus calculates and accumulates amounts ofchange ΔO2 in the oxygen storage amount as in mathematical expressions 7and 8, thereby determining the accumulated value at the time point t7 asa maximum oxygen storage amount CUFmax6.ΔO2=0.23×mfr×(abyfs−stoich)  [Expression 7]CUFmax6=ΣΔO2(time section t=t 6 to t 7)  [Expression 8]

Thus, in this embodiment, the maximum oxygen storage amount CUFmax6 isestimated on the basis of the output of the upstream-most air-fuel ratiosensor 66 (by utilizing (i.e., in consideration of) the fact that theupstream-of-first catalyst air-fuel ratio is controlled to the secondlean air-fuel ratio). Then, at a time point t7, the apparatus returnsthe air-fuel ratio of a mixture taken into the internal combustionengine to the stoichiometric air-fuel ratio. From the time point t7 on,the apparatus determines whether the upstream-most air-fuel ratio sensor66 has abnormality by determining whether mathematical expression 9 issatisfied.((vabyfsL−vabyfsR)/dvref)<α  [Expression 9]

In mathematical expression 9, dvref in the left side, as indicated inFIG. 3, is a deviation (positive constant value) between the value ofoutput vabyfs of the upstream-most air-fuel ratio sensor 66 occurringwhen the detected air-fuel ratio A/F of gas is the first lean air-fuelratio (stoich/0.98) and the value of output vabyfs of the upstream-mostair-fuel ratio sensor 66 occurring when the detected air-fuel ratio A/Fis the first rich air-fuel ratio (stoich/1.02) in the case where theupstream-most air-fuel ratio sensor 66 is normal. In mathematicalexpression 9, a in the right side is a criterion (positive constantvalue) for the determination regarding degradation of the upstream-mostair-fuel ratio sensor.

As degradation of the upstream-most air-fuel ratio sensor 66 progresses,there is a tendency as indicated by a two-dot chain line in FIG. 3, thatis, the amount of shift (absolute value thereof) of the value of outputof the upstream-most air-fuel ratio sensor 66 from the value of outputof the sensor 66 occurring when the air-fuel ratio A/F of gas is thestoichiometric air-fuel ratio with respect to the amount of shift of theactual air-fuel ratio A/F of the gas subjected to the detection by theupstream-most air-fuel ratio sensor 66 from the stoichiometric air-fuelratio (stoich) tends to decrease.

Therefore, the deviation (vabyfsL−vabyfsR) between the value of variablevabyfsL storing the value of output vabyfs of the upstream-most air-fuelratio sensor 66 occurring in the case where the upstream-of-firstcatalyst air-fuel ratio is controlled to the first lean air-fuel ratio,and the value of variable vabyfsR storing the value of output vabyfs ofthe upstream-most air-fuel ratio sensor 66 occurring in the case wherethe upstream-of-first catalyst air-fuel ratio is controlled to the firstrich air-fuel ratio decreases in accordance with progress of thedegradation of the upstream-most air-fuel ratio sensor 66. That is, theratio ((vabyfsL−vabyfsR)/dvref) of the value of deviation(vabyfsL−vabyfsR) to the value of deviation dvref decreases inaccordance with progress of the degradation of the upstream-mostair-fuel ratio sensor 66.

On the basis of the above-described principle, the apparatus determinesthat the upstream-most air-fuel ratio sensor 66 is abnormal (hasdegraded) if mathematical expression 9 holds, that is, if the ratio((vabyfsL−vabyfsR)/dvref) of the value of deviation (vabyfsL−vabyfsR) tothe value of deviation dvref is less than the criterion α fordetermination regarding degradation of the upstream-most air-fuel ratiosensor 66.

Then, the apparatus determines whether the first and second catalysts53, 54 have degraded as described below. First, with regard to the firstcatalyst 53, the apparatus determines whether the maximum oxygen storageamount CSCmax3 of the first catalyst 53 is less than or equal to a firstcatalyst degradation criterion CSCRdn, and whether the maximum oxygenstorage amount CSCmax5 of the first catalyst 53 is less than or equal toa first catalyst degradation criterion CSCRup, and whether a meanmaximum oxygen storage amount CSCmax of the first catalyst 53(=(CSCmax3+CSCmax5)/2), that is, a mean value of the maximum oxygenstorage amount CSCmax3 and the maximum oxygen storage amount CSCmax5, isless than or equal to a first catalyst degradation criterion CSCRave.

Then, the apparatus determines, in principle, that the first catalyst 53has degraded if fulfillment is achieved of any one of the condition thatthe maximum oxygen storage amount CSCmax3 is less than or equal to thefirst catalyst degradation criterion CSCRdn, the condition that themaximum oxygen storage amount CSCmax5 is less than or equal to the firstcatalyst degradation criterion CSCRup, and the condition that the meanmaximum oxygen storage amount CSCmax is less than or equal to the firstcatalyst degradation criterion CSCRave. It is also possible to adopt aconstruction in which it is determined that the first catalyst 53 hasdegraded if an arbitrary combination of two of the three conditions isfulfilled, or a construction in which it is determined that the firstcatalyst 53 has degraded if all three conditions are fulfilled. However,if it is determined that the upstream-most air-fuel ratio sensor 66 isabnormal, the apparatus avoids (i.e., is prohibited from) determiningthat the first catalyst 53 has degraded.

Next, with regard to the second catalyst 54, the apparatus determineswhether the maximum oxygen storage amount CUFmax4 of the second catalyst54 is less than or equal to a second catalyst degradation criterionCUFRdn, and whether the maximum oxygen storage amount CUFmax6 of thesecond catalyst 54 is less than or equal to a second catalystdegradation criterion CUFRup, and whether a mean maximum oxygen storageamount CUFmax of the second catalyst 54 (=(CUFmax4+CUFmax6)/2), that is,a mean value of the maximum oxygen storage amount CUFmax4 and themaximum oxygen storage amount CUFmax6, is less than or equal to a secondcatalyst degradation criterion CUFRave.

Then, the apparatus determines, in principle, that the second catalyst54 has degraded if fulfillment is achieved of any one of the conditionthat the maximum oxygen storage amount CUFmax4 is less than or equal tothe second catalyst degradation criterion CUFRdn, the condition that themaximum oxygen storage amount CUFmax6 is less than or equal to thesecond catalyst degradation criterion CUFRup, and the condition that themean maximum oxygen storage amount CUFmax is less than or equal to thesecond catalyst degradation criterion CUFRave. In this case, too, it ispossible to adopt a construction in which it is determined that thesecond catalyst 54 has degraded if an arbitrary combination of two ofthe three conditions is fulfilled, or a construction in which it isdetermined that the second catalyst 54 has degraded if all threeconditions are fulfilled. However, if it is determined that theupstream-most air-fuel ratio sensor 66 is abnormal, the apparatus avoids(i.e., is prohibited from) determining that the second catalyst 54 hasdegraded, as in the above-described case.

Furthermore, the apparatus determines whether the catalyst device as aunit of the first and second catalysts 53, 54 has degraded bydetermining whether mathematical expression 10 is satisfied. In thiscase, too, the apparatus avoids determining that the catalyst device hasdegraded, if it has been determined that the upstream-most air-fuelratio sensor 66 is abnormal.CSCmax+CUFmax≦CRave  [Expression 10]

In mathematical expression 10, CSCmax in the left side may be replacedby CSCmax3 or CSCmax5, and CUFmax may be replaced by CUFmax4 or CUFmax6.CRave in the right side is a reference value of the maximum oxygenstorage amount for the determination regarding degradation of thecatalyst device that is a unit of the first and second catalysts 53, 54(entire catalyst degradation criterion). Above described is a summary ofthe catalyst degradation determining method employed by the apparatus.

<Actual Operation>

Actual operations of the emissions control apparatus (and the catalystdegradation determining apparatus) constructed as described above willbe described below with reference to FIGS. 6 to 17 showing flowchartsillustrating routines (programs) executed by the CPU 71 of theelectronic control unit 70.

(Ordinary Air-Fuel Ratio Control)

The CPU 71 executes the routine of FIG. 6 for calculating the final fuelinjection amount Fi and commanding fuel injection, every time the crankangle of any one of the cylinders reaches a predetermined crank anglepreceding the intake stroke top dead center (e.g., BTDC 90° C.A).Therefore, when the crank angle of an arbitrary cylinder reaches thepredetermined crank angle, the CPU 71 starts the routine at step S600,and proceeds to step S605, in which the CPU 71 determines a basic fuelinjection amount Fbase for setting the internal combustion engineair-fuel ratio at the stoichiometric air-fuel ratio from a map on thebasis of the intake air amount AFM measured by the air flow meter 61 andthe engine rotation speed NE.

Subsequently in step S610, the CPU 71 sets, as a final fuel injectionamount Fi, the value obtained by adding a below-described air-fuel ratiofeedback correction amount DFi to the multiplication product of thebasic fuel injection amount Fbase and a factor K. The value of thefactor K is normally “1.00”, and is set at a predetermined value otherthan “1.00” when the air-fuel ratio is to be forcibly changed in orderto make a determination regarding catalyst degradation as describedabove. Subsequently in step S615, the CPU 71 commands the injector 39 toinject the final fuel injection amount Fi of fuel. After that, in stepS620, the CPU 71 sets the value obtained by adding the final fuelinjection amount Fi to the present fuel injection total amount mfr, as anew fuel injection accumulated amount mfr. The fuel injectionaccumulated amount mfr is used in the calculation of the oxygen storageamount described below. After that, in step S695, the CPU 71 temporarilyends the routine. Through the above-described operation, thefeedback-corrected final fuel injection amount Fi is injected into thecylinder that is about to undergo the intake stroke.

Next described will be calculation of the aforementioned air-fuel ratiofeedback correction amount DFi. The CPU71 executes the routineillustrated in FIG. 7 at every elapse of a predetermined time (i.e., atpredetermined time intervals). Therefore, when a predetermined timing isreached, the CPU 71 starts the routine at step S700, and proceeds tostep S705, in which the CPU 71 determines whether a feedback controlcondition is fulfilled. The air-fuel ratio feedback control condition isfulfilled, for example, in the case where the engine cooling watertemperature THW is higher than or equal to a first predeterminedtemperature, and where the amount of intake air (load) per rotation ofthe internal combustion engine is less than or equal to a predeterminedvalue, and where the upstream-most air-fuel ratio sensor 66 is normal,and where the value of a catalyst degradation determination executionflag XHAN described below is “0”. The catalyst degradation determinationexecution flag XHAN, when the value thereof is “1”, indicates that anair-fuel ratio control of forcibly changing the air-fuel ratio for thepurpose of determination regarding catalyst degradation is beingexecuted. When the value thereof is “0”, the flag XHAN indicates thatthe air-fuel ratio control for determination regarding catalystdegradation is not being executed.

If description is continued on the assumption that the air-fuel ratiocontrol condition is fulfilled, the CPU 71 makes a determination of“YES” in step S705, and proceeds to step S710. In step S710, the CPU 71computes a present upstream-side control-purposed air-fuel ratio abyfs1of the first catalyst 53 by converting the sum (vabyfs+vafsfb) of thepresent output vabyfs of the upstream-most air-fuel ratio sensor 66 anda below-described subsidiary feedback control amount vafsfb on the basisof the map indicated in FIG. 3.

Subsequently in step S715, the CPU 71 computes a cylinder fuel supplyFc(k-N) provided N number of strokes (N number of intake strokes) priorto the present time point by dividing the cylinder intake air amountMc(k-N), that is, the amount of air taken into the cylinder thatunderwent the intake stroke at the time of N number of strokes prior tothe present time point, by the aforementioned upstream-sidecontrol-purposed air-fuel ratio abyfs1. The value N varies depending onthe amount of exhaust gas from the internal combustion engine, thedistance from the combustion chamber 25 to the upstream-most air-fuelratio sensor 66, etc.

The reason why the cylinder intake air amount Mc(k-N) provided N numberof strokes prior to the present time point is divided by theupstream-side control-purposed air-fuel ratio abyfs1 in order todetermine the cylinder fuel supply Fc(k-N) provided N number of strokesprior to the present time point is that a time corresponding to N numberof strokes is needed for the mixture burned in the combustion chamber 25to reach the upstream-most air-fuel ratio sensor 66. The cylinder intakeair amount Mc is computed at every intake stroke of each cylinder on thebasis of the then-occurring output AFM of the air flow meter 61 and theengine rotation speed NE (for example, the amount Mc is computed bydividing a value obtained by performing a primary delay process on theoutput AFM of the air flow meter 61 by the engine rotation speed NE),and is stored in the RAM 73 corresponding to each intake stroke.

Subsequently in step S720, the CPU 71 computes a target cylinder fuelsupply Fcr(k-N) for the time of N number of strokes prior to the presenttime point by dividing the cylinder intake air amount Mc(k-N) provided Nnumber of strokes prior to the present time point by the target air-fuelratio abyfr(k-N) provided at the time point of N number of strokes priorto the present time point (stoichiometric air-fuel ratio stoich on thisside). Then, in step S725, the CPU 71 sets the value obtained bysubtracting the cylinder fuel supply Fc(k-N) from the target cylinderfuel supply Fcr(k-N), as a cylinder fuel supply deviation DFc. That is,the cylinder fuel supply deviation DFc indicates the excess or shortfallof fuel resulting from the supply of fuel into the cylinder at the timepoint of N number of strokes before. Subsequently in step S730, the CPU71 computes an air-fuel ratio feedback correction amount DFi as inmathematical expression 11.DFi=(Gp×DFc+Gi×SDFc)×KFB  [Expression 11]

In mathematical expression 11, Gp is a pre-set proportional gain, and Giis a pre-set integral gain. Furthermore, in mathematical expression 11,although the factor KFB is preferably variable depending on the enginerotation speed NE, the cylinder intake air amount Mc, etc., the factorKFB is set at “1” in this embodiment. Still further, the value SDFc isan integrated value of the cylinder fuel supply deviation DFc, and isupdated in step S735. That is, in step S735, the CPU 71 adds thecylinder fuel supply deviation DFc determined in step S725 to thepresent integrated value SDFc of the cylinder fuel supply deviation DFc,thereby determining a new integrated value SDFc of the cylinder fuelsupply deviation. Subsequently in step S795, the CPU 71 temporarily endsthe routine.

Thus, the air-fuel ratio feedback correction amount DFi is determined bya proportional-plus-integral control. Since the air-fuel ratio feedbackcorrection amount DFi is reflected in the fuel injection amount in stepS610 and step S615 in FIG. 6, the excess or shortfall of fuel supplyoccurring N number of strokes prior to the present time point, and themean value of air-fuel ratio is made substantially equal to the targetair-fuel ratio abyfr.

If it is determined in step S705 that the air-fuel ratio feedbackcontrol condition is not fulfilled, the CPU 71 makes a determination of“NO” in step S705, and proceeds to step S740. In step S740, the CPU 71sets the value of air-fuel ratio feedback correction amount DFi at “0”.Subsequently in step S795, the CPU 71 temporarily ends the routine.Thus, in the case where the air-fuel ratio feedback control condition isnot fulfilled (including a case where the catalyst degradationdetermination is being executed), the CPU 71 sets the air-fuel ratiofeedback correction amount DFi at “0”, and avoids correction of theair-fuel ratio (basic fuel injection amount Fbase).

Next described will be an air-fuel ratio feedback control based on theoutput Voxs1 of the downstream-of-first catalyst air-fuel ratio sensor67. This control is also referred to as “subsidiary feedback control”.By the subsidiary feedback control, a subsidiary feedback control amountvafsfb is calculated.

In order to determine the subsidiary feedback control amount vafsfb, theCPU 71 executes a routine illustrated in FIG. 8 at every elapse of apredetermined time. Therefore, when a predetermined timing is reached,the CPU 71 starts the routine at step S800, and proceeds to step S805,in which the CPU 71 determines whether a subsidiary feedback controlcondition is fulfilled. The subsidiary feedback control condition isfulfilled, for example, in a case where the engine cooling watertemperature THW is higher than or equal to a second predeterminedtemperature that is higher than the first predetermined temperature, andwhere the downstream-of-first catalyst air-fuel ratio sensor 67 isnormal, in addition to fulfillment of the feedback control condition instep S705.

If description is continued on the assumption that the subsidiaryfeedback control condition is fulfilled, the CPU 71 makes adetermination of “YES” in step S805, and proceeds to step S810. In stepS810, the CPU 71 computes an output deviation DVoxs by subtracting thepresent output Voxs1 of the downstream-of-first catalyst air-fuel ratiosensor 67 from a predetermined target value Voxsref. The target valueVoxsref is determined so that the emissions control efficiency of thefirst catalyst 53 becomes good (optimal). In this embodiment, the targetvalue Voxsref is set at a value corresponding to the stoichiometricair-fuel ratio. Subsequently in step S815, the CPU 71 computes asubsidiary feedback control amount vafsfb as in mathematical expression12.vafsfb=Kp×DVoxs+Ki×SDVoxs  [Expression 12]

In mathematical expression 12, Kp is a pre-set proportional gain, and Kiis a pre-set integral gain. Furthermore, the value SDVoxs is anintegrated value of output deviation DVoxs, and is updated in step S820.That is, in step S820, the CPU 71 adds the output deviation DVoxsdetermined in step S810 to the present integrated value SDVoxs of theoutput deviation, thereby determining a new integrated value SDVoxs ofthe output deviation. Subsequently in step S895, the CPU 71 temporarilyends the routine.

Thus, the subsidiary feedback control amount vafsfb is determined. Thisvalue is added to an actual output of the upstream-most air-fuel ratiosensor 66 in step S710 in FIG. 7, and the sum (vabyfs+vafsfb) isconverted into the upstream-side control-purposed air-fuel ratio abyfs1on the basis of the map indicated in FIG. 3. That is, the upstream-sidecontrol-purposed air-fuel ratio abyfs1 determined on the basis of theoutput Voxs1 of the downstream-of-first catalyst air-fuel ratio sensor67 is determined as an air-fuel ratio that differs from the air-fuelratio actually detected by the upstream-most air-fuel ratio sensor 66,by an amount corresponding to the subsidiary feedback control amountvafsfb.

As a result, since the cylinder fuel supply Fc(k-N) calculated in stepS715 in FIG. 7 changes in accordance with the output Voxs1 of thedownstream-of-first catalyst air-fuel ratio sensor 67, the air-fuelratio feedback correction amount DFi is changed in accordance with theoutput Voxs1 of the downstream-of-first catalyst air-fuel ratio sensor67 in steps S725 and S730. Thus, the engine air-fuel ratio is controlledso that the air-fuel ratio of gas downstream of the first catalyst 53becomes equal to the target value Voxsref.

For example, if the output Voxs1 of the downstream-of-first catalystair-fuel ratio sensor 67 indicates a value corresponding to an air-fuelratio that is lean of the stoichiometric air-fuel ratio as an averageair-fuel ratio that is on the lean side of the stoichiometric air-fuelratio, the output deviation DVoxs determined in step S810 is a positivevalue, and therefore the subsidiary feedback control amount vafsfbbecomes a positive value. Therefore, the upstream-side control-purposedair-fuel ratio abyfs1 determined in step S710 is determined as a valuethat is lean of (greater than) the air-fuel ratio actually detected bythe upstream-most air-fuel ratio sensor 66. Hence, the cylinder fuelsupply Fc(k-N) determined in step S715 becomes a small value, and thecylinder fuel supply deviation DFc is determined as a great value.Therefore, the air-fuel ratio feedback correction amount DFi becomes agreat positive value. Therefore, the final fuel injection amount Fidetermined in step S610 in FIG. 6 becomes greater than the basic fuelinjection amount Fbase, and a control is performed so that the engineair-fuel ratio becomes rich of the stoichiometric air-fuel ratio.

Conversely, if the output Voxs1 of the downstream-of-first catalystair-fuel ratio sensor 67 indicates a value corresponding to an air-fuelratio that is rich of the stoichiometric air-fuel ratio as an averageengine air-fuel ratio that is on the rich side of the stoichiometricair-fuel ratio, the output deviation DVoxs determined in step S810 is anegative value, and therefore the subsidiary feedback control amountvafsfb becomes a negative value. Therefore, the upstream-sidecontrol-purposed air-fuel ratio abyfs1 determined in step S710 isdetermined as a value that is rich of (less than) the air-fuel ratioactually detected by the upstream-most air-fuel ratio sensor 66. Hence,the cylinder fuel supply Fc(k-N) determined in step S715 becomes a greatvalue, and the cylinder fuel supply deviation DFc is determined as anegative value. Therefore, the air-fuel ratio feedback correction amountDFi becomes a negative value. Therefore, the final fuel injection amountFi determined in step S610 in FIG. 6 becomes less than the basic fuelinjection amount Fbase, and a control is performed so that the engineair-fuel ratio becomes lean of the stoichiometric air-fuel ratio.

Thus, a control is performed so that the air-fuel ratio of gasdownstream of the first catalyst 53 becomes very close to thestoichiometric air-fuel ratio. Therefore, emissions are continuouslylessened even in a case where the first and second catalysts 53, 54 havedegraded and the maximum oxygen storage amount CSCmax and the maximumoxygen storage amount CUFmax have decreased.

If it is determined in step S805 that the subsidiary feedback controlcondition is not fulfilled, the CPU 71 makes a determination of “NO” instep S805, and proceeds to step S825. In step S825, the CPU 71 sets thevalue of subsidiary feedback control amount vafsfb at “0”. Subsequentlyin step S895, the CPU 71 temporarily ends the routine. Thus, in the casewhere the subsidiary feedback control condition is not fulfilled(including a case where an air-fuel ratio switching control is beingexecuted), the CPU 71 sets the subsidiary feedback control amount vafsfbat “0”, and avoids correction of the air-fuel ratio feedback correctionamount DFi (upstream-side control-purposed air-fuel ratio abyfs1) basedon the output Voxs1 of the downstream-of-first catalyst air-fuel ratiosensor 67. The ordinary air-fuel ratio control is executed in theabove-described manner.

(Air-Fuel Ratio Control for Determination Regarding CatalystDegradation)

Next described will be an air-fuel ratio control for determinationregarding catalyst degradation. The CPU 71 executes each of the routinesillustrated by the flowcharts of FIG. 9 to FIG. 15 at every elapse of apredetermined time.

When a predetermined timing is reached, the CPU 71 starts the routine instep S900 in FIG. 9, and proceeds to step S905, in which the CPU 71determines whether the value of the catalyst degradation determinationexecution flag XHAN is “0”. If description is continued on theassumption that the air-fuel ratio control for determination regardingcatalyst degradation is not being executed, and that a catalystdegradation determining condition is not fulfilled, the value of thecatalyst degradation determination execution flag XHAN is “0”.Therefore, the CPU 71 makes a determination of “YES” in step S905, andproceeds to step S910, in which the CPU 71 sets the value of the factorK used in step S610 in FIG. 6 at 1.00.

Subsequently in step S915, the CPU 71 determines whether the catalystdegradation determining condition is fulfilled. The catalyst degradationdetermining condition is fulfilled in the case where the cooling watertemperature THW is higher than or equal to a predetermined temperature,and where the vehicle speed acquired via a vehicle speed sensor (notshown) is higher than or equal to a predetermined vehicle speed, andwhere the internal combustion engine is in a steady operation with theper-unit-time amount of change in the throttle valve opening TA beingless than or equal to a predetermined amount. It is also possible to addto the catalyst degradation determining condition, any one or two ormore of a condition that a predetermined time has elapsed following theprevious catalyst degradation determination, a condition that thevehicle has traveled at least a predetermined distance following theprevious catalyst degradation determination, and a condition that theinternal combustion engine 10 has been operated for at least apredetermined time following the previous catalyst degradationdetermination. At the present stage, the catalyst degradationdetermining condition is not fulfilled as mentioned above. Therefore,the CPU 71 makes a determination of “NO” in step S915, and proceeds tostep S995, in which the CPU 71 temporarily ends the routine.

Description will be further continued on the assumption that thecatalyst degradation determining condition is fulfilled although theair-fuel ratio control for determination regarding catalyst degradationis not performed at the present time point as in the case of the timepoint t1 in FIG. 5. In this case, the CPU 71 makes a determination of“YES” in step S905, and proceeds to step S910, in which the CPU 71 setsthe value of the factor K at 1.00. Subsequently in step S915, the CPU 71makes a determination of “YES” since the catalyst degradationdetermining condition is fulfilled. Subsequently in step S920, the CPU71 sets the value of the catalyst degradation determination executionflag XHAN to “1”.

Subsequently in step S925, the CPU 71 sets the value of Mode at “1” inorder to enter the first mode. Subsequently in step S930, the CPU 71sets the value of the factor K at 0.98. Then, in step S995, the CPU 71temporarily ends the routine. As a result, the aforementioned air-fuelratio feedback control condition becomes unfulfilled. Therefore, the CPU71 makes a determination of “NO” in step S705 in FIG. 7, and proceeds tostep S740, in which the air-fuel ratio feedback correction amount DFi isset at “0”. As a result, due to execution of step S610 in FIG. 6, thevalue obtained by multiplying basic fuel injection amount Fbase by 0.98is computed as a final fuel injection amount Fi. Since thethus-determined final fuel injection amount Fi is injected, the internalcombustion engine air-fuel ratio is controlled to the first leanair-fuel ratio that is lean of the stoichiometric air-fuel ratio.

After that, the CPU 71 repeatedly executes the processing of the routineof FIG. 9 starting at step S900. However, since the value of thecatalyst degradation determination execution flag XHAN is “1”, the CPU71 makes a determination of “NO” in step S905, and immediately proceedsto step S995, in which the CPU 71 temporarily ends the routine.

In the mean time, the CPU 71 executes a first mode control routineillustrated in FIG. 10 at every elapse of a predetermined time.Therefore, when a predetermined timing is reached, the CPU 71 starts theroutine at step S1000, and proceeds to step S1005, in which the CPU 71determines whether the value of Mode “1”. Since in this case, the valueof Mode has been set at “1” in step S925 in FIG. 9, the CPU 71 makes adetermination of “YES” in step S1005, and proceeds to step S1005. Instep S1010, the CPU 71 determines whether the output Voxs1 of thedownstream-of-first catalyst air-fuel ratio sensor has changed from avalue indicating an air-fuel ratio rich of the stoichiometric air-fuelratio to a value indicating an air-fuel ratio lean of the stoichiometricair-fuel ratio. Since at the present time point, the internal combustionengine air-fuel ratio has just been changed to the first lean air-fuelratio, the output Voxs1 of the downstream-of-first catalyst air-fuelratio sensor indicates an air-fuel ratio rich of the stoichiometricair-fuel ratio. Therefore, the CPU 71 makes a determination of “NO” instep S1010, and proceeds to step S1095, in which the CPU 71 temporarilyends the routine.

After that, the CPU 71 repeatedly executes steps S1000 to S1010 in FIG.10. Since the air-fuel ratio is kept at the first lean air-fuel ratio,the downstream-of-first catalyst air-fuel ratio sensor output Voxs1changes from the rich air-fuel ratio-indicating value to a lean air-fuelratio-indicating value at the elapse of a predetermined time, as in thecase of the time point t2 in FIG. 5. Therefore, when the CPU 71 proceedsto step S1010, the CPU 71 makes a determination of “YES”. Then, the CPU71 proceeds to step S1015, in which the CPU 71 stores the value ofoutput vabyf of the upstream-most air-fuel ratio sensor 66 occurring atthe present time point into the variable VabyfsL. Subsequently in stepS1020, the CPU 71 sets the value of Mode at “2” so as to enter thesecond mode. Subsequently in step S1025, the CPU 71 computes and sets avalue of the factor K on the basis of the below-described maximum oxygenstorage amount Cmaxall of the entire catalyst device estimated in theprevious catalyst degradation determination operation and a tableindicated in the box of step S1025 in FIG. 10. Subsequently in stepS1095, the CPU 71 temporarily ends the routine.

Therefore, the factor K is changed in accordance with the value ofmaximum oxygen storage amount Cmaxall of the entire catalyst device.That is, the factor K is set (relatively small) so as to approach 0.98(i.e., the value of the factor K for achieving the first lean air-fuelratio) as the value of maximum oxygen storage amount Cmaxall increases,and is set (relatively great) so as to approach 1.00 (i.e., the value ofthe factor K for achieving the stoichiometric air-fuel ratio) as thevalue of maximum oxygen storage amount Cmaxall decreases. That is, instep S1025, the factor K is set at an arbitrary value that is greaterthan 0.98 and is less than 1.00 in accordance with the value of themaximum oxygen storage amount Cmaxall of the entire catalyst device.

Therefore, due to the execution of step S610 in FIG. 6, the valueobtained by multiplying the basic fuel injection amount Fbase by thefactor K is calculated as a final fuel injection amount Fi. Since thisfinal fuel injection amount Fi is injected, the internal combustionengine air-fuel ratio is controlled to the second lean air-fuel ratiothat is lean of the stoichiometric air-fuel ratio and rich of the firstlean air-fuel ratio, and that is changed in accordance with the value ofthe maximum oxygen storage amount Cmaxall of the entire catalyst device(approaches the stoichiometric air-fuel ratio as the maximum oxygenstorage amount Cmaxall decreases).

When the second mode (Mode=2) is entered, the CPU 71 executes a similarmode control. Then, the CPU 71 sequentially changes the mode from thethird mode to the fourth, fifth and sixth modes, and executes a controlcorresponding to each mode. Briefly, in the second mode whose routine isillustrated by a flowchart of FIG. 11, after starting in step S1100, instep S1105, the CPU 71 determines whether the value of Mode is “2”. Ifthe value of Mode is “2”, the process proceeds from step S1105 to stepS1110. In step S1110, the CPU 71 monitors whether the output Voxs2 ofthe downstream-of-second catalyst air-fuel ratio sensor 68 has changedfrom a value indicating an air-fuel ratio that is rich of thestoichiometric air-fuel ratio to a value indicating an air-fuel ratiothat is lean of the stoichiometric air-fuel ratio.

If the output Voxs2 of the downstream-of-second catalyst air-fuel ratiosensor 68 changes from a value indicating an air-fuel ratio rich of thestoichiometric air-fuel ratio to a value indicating an air-fuel ratiolean of the stoichiometric air-fuel ratio as indicated at the time pointt3 in FIG. 5, the CPU 71 proceeds to step S1115, in which the CPU 71sets the value of Mode at “3” so as to enter the third mode.Subsequently in step S1120, the CPU 71 sets the value of the factor K at1.02. As a result, the internal combustion engine air-fuel ratio iscontrolled to the first rich air-fuel ratio that is rich of thestoichiometric air-fuel ratio. The routine ends in step S1195.

Similarly, in the third mode whose routine is illustrated by theflowchart of FIG. 12, after starting in step S1200, the CPU 71determines in step S1205 whether the value of Mode is “3”. If the valueof Mode is “3”, the CPU 71 proceeds from step S1205 to step S1210. Instep S1210, the CPU 71 monitors whether the output Voxs1 of thedownstream-of-first catalyst air-fuel ratio sensor 67 has changed from avalue indicating an air-fuel ratio that is lean of the stoichiometricair-fuel ratio to a value indicating an air-fuel ratio that is rich ofthe stoichiometric air-fuel ratio.

If the output Voxs1 of the downstream-of-first catalyst air-fuel ratiosensor 67 changes from a value indicating an air-fuel ratio lean of thestoichiometric air-fuel ratio to a value indicating an air-fuel ratiorich of the stoichiometric air-fuel ratio as indicated at the time pointt4 in FIG. 5, the CPU 71 proceeds from step S1210 to step S1215. In stepS1215, the CPU 71 stores the present value of output vabyfs of theupstream-most air-fuel ratio sensor 66 into the variable VabyfsR.Subsequently in step S1220, the CPU 71 sets the value of Mode at “4” soas to enter the fourth mode. Subsequently in step S1225, the CPU 71calculates and sets a value of the factor K on the basis of thebelow-described maximum oxygen storage amount Cmaxall of the entirecatalyst device estimated during the previous catalyst degradationdetermination and a table indicated in the box of step S1225 in FIG. 12.Subsequently in step S1295, the CPU 71 temporarily ends the routine.

Therefore, the factor K is changed in accordance with the value ofmaximum oxygen storage amount Cmaxall of the entire catalyst device.That is, the factor K is set (relatively great) so as to approach 1.02(i.e., the value of the factor K for achieving the first rich air-fuelratio) as the value of maximum oxygen storage amount Cmaxall increases,and is set (relatively small) so as to approach 1.00 (i.e., the value ofthe factor K for achieving the stoichiometric air-fuel ratio) as thevalue of maximum oxygen storage amount Cmaxall decreases. That is, instep S1225, the factor K is set at an arbitrary value that is greaterthan 1.00 and is less than 1.02 in accordance with the value of themaximum oxygen storage amount Cmaxall of the entire catalyst device. Asa result, the internal combustion engine air-fuel ratio is controlled tothe second rich air-fuel ratio that is rich of the stoichiometric andlean of the first rich air-fuel ratio, and that is changed in accordancewith the value of the maximum oxygen storage amount Cmaxall of theentire catalyst device (approaches the stoichiometric air-fuel ratio asthe maximum oxygen storage amount Cmaxall decreases).

Similarly, in the fourth mode whose routine is illustrated by theflowchart of FIG. 13, after starting in step S1300, the CPU 71determines in step S1305 whether the value of Mode is “4”. If the valueof Mode is “4”, the CPU 71 proceeds from step S1305 to step S1310. Instep S1310, the CPU 71 monitors whether the output Voxs2 of thedownstream-of-second catalyst air-fuel ratio sensor 68 has changed froma value indicating an air-fuel ratio that is lean of the stoichiometricair-fuel ratio to a value indicating an air-fuel ratio that is rich ofthe stoichiometric air-fuel ratio.

If the output Voxs2 of the downstream-of-second catalyst air-fuel ratiosensor 68 changes from a value indicating an air-fuel ratio lean of thestoichiometric air-fuel ratio to a value indicating an air-fuel ratiorich of the stoichiometric air-fuel ratio as indicated at the time pointt5 in FIG. 5, the CPU 71 proceeds from step S1310 to step S1315. In stepS1315, the CPU 71 sets the value of Mode at “5” so as to enter the fifthmode. Subsequently in step S1320, the CPU 71 sets the value of thefactor K at 0.98. As a result, the internal combustion engine air-fuelratio is controlled to the first lean air-fuel ratio. The routine endsin step S1395.

Similarly, in the fifth mode whose routine is illustrated by theflowchart of FIG. 14, after starting in step S1400, the CPU 71determines in step S1405 whether the value of Mode is “5”. If the valueof Mode is “5”, the CPU 71 proceeds from step S1405 to step S1410. Instep S1410, the CPU 71 monitors whether the output Voxs1 of thedownstream-of-first catalyst air-fuel ratio sensor 67 has changed from avalue indicating an air-fuel ratio that is rich of the stoichiometricair-fuel ratio to a value indicating an air-fuel ratio that is lean ofthe stoichiometric air-fuel ratio.

If the output Voxs1 of the downstream-of-first catalyst air-fuel ratiosensor 67 changes from a value indicating an air-fuel ratio rich of thestoichiometric air-fuel ratio to a value indicating an air-fuel ratiolean of the stoichiometric air-fuel ratio as indicated at the time pointt6 in FIG. 5, the CPU 71 proceeds from step S1410 to step S1415. In stepS1415, the CPU 71 sets the value of Mode at “6” so as to enter the sixthmode. Subsequently in step S1420, the CPU 71 calculates and sets a valueof the factor K (0.98<K<1.00) on the basis of the below-describedmaximum oxygen storage amount Cmaxall of the entire catalyst deviceestimated during the previous catalyst degradation determination and atable indicated in the box of step S1420 that is the same as theaforementioned table indicated in the box of step S1025. As a result,the internal combustion engine air-fuel ratio is controlled to thesecond lean air-fuel ratio. The routine ends in step S1495.

Similarly, in the sixth mode whose routine is illustrated by theflowchart of FIG. 15, after starting in step S1500, the CPU 71determines in step S1505 whether the value of Mode is “6”. If the valueof Mode is “6”, the CPU 71 proceeds from step S1505 to step S1510. Instep S1510, the CPU 71 monitors whether the output Voxs2 of thedownstream-of-second catalyst air-fuel ratio sensor 68 has changed froma value indicating an air-fuel ratio that is rich of the stoichiometricair-fuel ratio to a value indicating an air-fuel ratio that is lean ofthe stoichiometric air-fuel ratio.

If the output Voxs2 of the downstream-of-second catalyst air-fuel ratiosensor 68 changes from a value indicating an air-fuel ratio rich of thestoichiometric air-fuel ratio to a value indicating an air-fuel ratiolean of the stoichiometric air-fuel ratio as indicated at the time pointt7 in FIG. 5, the CPU 71 proceeds from step S1510 to step S1515. In stepS1515, the CPU 71 sets the value of Mode at “0”. Subsequently in stepS1520, the CPU 71 sets the value of the catalyst degradationdetermination execution flag XHAN at “0”. Then, in step S1595, the CPU71 temporarily ends the routine. Therefore, when executing the routineof FIG. 9, the CPU 71 makes a determination of “YES” at step S905, andproceeds to step S910. Thus, the value of the factor K is returned to1.00. Furthermore, if the other air-fuel ratio feedback controlcondition and the other subsidiary feedback control condition arefulfilled, the CPU 71 makes a determination of “YES” at step S705 andstep S805, so that the air-fuel ratio feedback control and thesubsidiary feedback control are started again.

As described above, if the catalyst degradation determining condition isfulfilled, the internal combustion engine air-fuel ratio is forciblycontrolled to the first lean air-fuel ratio that is constant, the secondlean air-fuel ratio that changes in accordance with the value of themaximum oxygen storage amount Cmaxall of the entire catalyst deviceestimated during the previous catalyst degradation determinationoperation (approaches the stoichiometric air-fuel ratio as the maximumoxygen storage amount Cmaxall decreases), the first rich air-fuel ratiothat is constant, the second rich air-fuel ratio that changes inaccordance with the value of the maximum oxygen storage amount Cmaxallof the entire catalyst device estimated during the previous catalystdegradation determination operation (approaches the stoichiometricair-fuel ratio as the maximum oxygen storage amount Cmaxall decreases),the first lean air-fuel ratio, and the second lean air-fuel ratio, inthat order.

(Estimation of Oxygen Storage Amount, Determination RegardingAbnormality of Upstream-Most Air-Fuel Ratio Sensor, Catalyst DegradationDetermination)

Next described will be operations for estimation of a maximum oxygenstorage amount for determination regarding catalyst degradation,determination regarding abnormality of the upstream-most air-fuel ratiosensor 66, and determination regarding catalyst degradation based on theestimated maximum oxygen storage amount. The CPU 71 executes routinesillustrated by the flowcharts of FIGS. 16 and 17 at every elapse of apredetermined time.

Therefore, when a predetermined timing is reached, the CPU 71 startsprocessing at step S1600 in FIG. 16, and proceeds to step S1605, inwhich the CPU 71 computes an amount of change ΔO2 in the oxygen storageamount as in mathematical expression 13.ΔO2=0.23×mfr×(abyfs−stoich)  [Expression 13]

Subsequently in step S1610, the CPU 71 determines whether the value ofMode is “3”. If the value of Mode is “3”, the CPU 71 makes adetermination of “YES” at step S1610, and proceeds to step S1615. Instep S1615, the CPU 71 sets the value obtained by adding the absolutevalue of the amount of change ΔO2 in oxygen storage amount to thepresent value of the oxygen storage amount OSA3 of the third mode as anew oxygen storage amount OSA3. Then, the CPU 71 proceeds to step S1650.The reason for adding the absolute value of the amount of change ΔO2 inoxygen storage amount is that mathematical expression 13 provides anegative value of the amount of change ΔO2 in oxygen storage amount ofthe third mode, as is apparent from comparison between mathematicalexpression 13 and mathematical expression 1.

This process (step S1600 to step S1615) is repeatedly executed as longas the value of Mode is “3”. As a result, during the third mode (Mode=3)where the air-fuel ratio of gas upstream of the first catalyst 53 is setat the first rich air-fuel ratio, the oxygen storage amount OSA3 of thefirst catalyst 53 is calculated. If the CPU 71 makes a determination of“NO” at step S1610, the CPU 71 immediately proceeds from step S1610 tostep S1620.

If the CPU 71 proceeds to step S11620, the CPU 71 determines whether thevalue of Mode is “4”. If the value of Mode is “4”, the CPU 71 makes adetermination of “YES” in step S1620, and proceeds to step S1625. Instep S1625, the CPU 71 sets the value obtained by adding the absolutevalue of the amount of change ΔO2 in oxygen storage amount to thepresent oxygen storage amount OSA4 of the fourth mode as a new oxygenstorage amount OSA4. Then, the CPU 71 proceeds to step S1650. The reasonfor adding the absolute value of the amount of change ΔO2 in oxygenstorage amount is that mathematical expression 13 provides a negativevalue of the amount of change ΔO2 in the oxygen storage amount of thefourth mode as is apparent from comparison between mathematicalexpression 13 and mathematical expression 3.

The above-described process (steps S1600, 1605, 1610, 1620, 1625) isrepeatedly executed as long as the value of Mode is “4”. As a result,the oxygen storage amount OSA4 of the second catalyst 54 is calculatedin the fourth mode (Mode=4) where the air-fuel ratio of gas upstream ofthe first catalyst 53 is set at the second rich air-fuel ratio. If theCPU 71 makes a determination of “NO” in step S1620, the CPU 71immediately proceeds from step S1620 to step S1630.

Similarly, if the CPU 71 proceeds to step S1630, the CPU 71 determineswhether the value of Mode is “5”. If the value of Mode is “5”, the CPUproceeds to step S1635. In step S1635, the CPU 71 sets the valueobtained by adding the amount of change ΔO2 in oxygen storage amount tothe present oxygen storage amount OSA5 of the fifth mode as a new oxygenstorage amount OSA5. Then, the CPU 71 proceeds to step S1650.

The above-described process (steps S1600, 1605, 1610, 1620, 1630, 1635)is repeatedly executed as long as the value of Mode is “5”. As a result,the oxygen storage amount OSA5 of the first catalyst 53 is calculated inthe fifth mode (Mode=5) where the air-fuel ratio of gas upstream of thefirst catalyst 53 is set at the first lean air-fuel ratio. If the CPU 71makes a determination of “NO” in step S1630, the CPU 71 immediatelyproceeds from step S1630 to step S1640.

Similarly, if the CPU 71 proceeds to step S1640, the CPU 71 determineswhether the value of Mode is “6”. If the value of Mode is “6”, the CPUproceeds to step S1645. In step S1645, the CPU 71 sets the valueobtained by adding the amount of change ΔO2 in oxygen storage amount tothe present oxygen storage amount OSA6 of the sixth mode as a new oxygenstorage amount OSA6. Then, the CPU 71 proceeds to step S1650.

The above-described process (steps S1600, 1605, 1610, 1620, 1630, 1640,1645) is repeatedly executed as long as the value of Mode is “6”. As aresult, the oxygen storage amount OSA6 of the second catalyst 54 iscalculated in the sixth mode (Mode=6) where the air-fuel ratio of gasupstream of the first catalyst 53 is set at the second lean air-fuelratio. If the CPU 71 makes a determination of “NO” in step S1640, theCPU 71 immediately proceeds from step S1640 to step S1650.

When the CPU 71 proceeds to step S1650, the CPU 71 sets the total amountmfr of the fuel injection amount Fi at “0” in step S1650. Subsequentlyin step S1695, the CPU 71 temporarily ends the routine.

Furthermore, the CPU 71 executes a routine for determination regardingabnormality of the upstream-most air-fuel ratio sensor 66 anddetermination regarding catalyst degradation at every elapse of apredetermined time. Therefore, when a predetermined timing is reached,the CPU 71 starts processing at step S1700, and proceeds to step S1702,in which the CPU 71 determines whether the value of the catalystdegradation determination execution flag XHAN has changed from “1” to“0”. If at this time, the sixth mode ends and the value of the catalystdegradation determination execution flag XHAN is changed to “0” in stepS1520 in FIG. 15, the CPU 71 makes a determination of “YES” in stepS1702, and proceeds to step S1704. Conversely, if the value of thecatalyst degradation determination execution flag XHAN has not changed,the CPU 71 immediately proceeds from step S1702 to step S1795, in whichthe CPU 71 temporarily ends the routine.

If it is assumed that the sixth mode has just ended, the value of thecatalyst degradation determination execution flag XHAN has just beenchanged from “1” to “0”, and therefore, the CPU 71 proceeds from stepS1702 to step S1704, in which the present oxygen storage amounts OSA3,OSA4, OSA5, OSA6 are stored as maximum oxygen storage amounts CSCmax3(the first maximum oxygen storage amount of the first catalyst), CUFmax4(the first maximum oxygen storage amount of the second catalyst),CSCmax5 (the second maximum oxygen storage amount of the firstcatalyst), and CUFmax6 (the second maximum oxygen storage amount of thesecond catalyst).

Subsequently in step S1706, on the basis of the value of variableVabyfsL, the value of variable VabyfsR, and mathematical expression 9(mathematical expression shown in the box of step S1706), the CPU 71determines whether the ratio ((vabyfsL−vabyfsR)/dvref) of the value ofdeviation (vabyfsL−vabyfsR) to the value of deviation dvref is less thana criterion α for determination regarding degradation of theupstream-most air-fuel ratio sensor. If the ratio((vabyfsL−vabyfsR)/dvref) is less than the upstream-most air-fuel ratiosensor degradation criterion α, the CPU 71 proceeds to step S1708 andsets the value of an upstream-most air-fuel ratio sensor abnormalitydetermination result flag XSENR at “1”, thereby indicating that theupstream-most air-fuel ratio sensor 66 is abnormal (degraded). It is tobe noted herein that step S1706 functions as an upstream-of-catalystair-fuel ratio sensor abnormality detecting means.

Conversely, if it is determined in step S1706 that the ratio((vabyfsL−vabyfsR)/dvref) is greater than or equal to the upstream-mostair-fuel ratio sensor degradation criterion α, the CPU 71 proceeds tostep S1710. In step S1710, the CPU 71 sets the value of theupstream-most air-fuel ratio sensor abnormality flag XSENR to “0”,thereby indicating that the upstream-most air-fuel ratio sensor 66 isnormal (not degraded).

Subsequently in step S1712, the CPU 71 stores a mean value of themaximum oxygen storage amount CSCmax3 and the maximum oxygen storageamount CSCmax5 as a mean maximum oxygen storage amount CSCmax of thefirst catalyst 53.

Subsequently in step S1714, the CPU 71 determines whether the meanmaximum oxygen storage amount CSCmax is less than or equal to the firstcatalyst degradation criterion CSCRave. If the mean maximum oxygenstorage amount CSCmax is less than or equal to the first catalystdegradation criterion CSCRave, the CPU 71 proceeds to step S1716, inwhich the CPU 71 determines whether the value of the upstream-mostair-fuel ratio sensor abnormality flag XSENR is “0”.

If it is determined in step S1716 that the value of the upstream-mostair-fuel ratio sensor abnormality flag XSENR is “0”, the CPU 71 sets thevalue of a first catalyst degradation determination result flag XSCR to“1” in step S1718, thereby indicating that the first catalyst 53 hasdegraded. Conversely, if it is determined in step S1716 that the valueof the upstream-most air-fuel ratio sensor abnormality flag XSENR is not“0” (i.e., if the value is “1”), the CPU 71 sets the value of the firstcatalyst degradation determination result flag XSCR at “2” in stepS1720, thereby indicating that determination regarding degradation ofthe first catalyst 53 has not been made.

If it is determined in step S1714 that the mean maximum oxygen storageamount CSCmax is greater than the first catalyst degradation criterionCSCRave, the CPU 71 sets the value of the first catalyst degradationdetermination result flag XSCR at “0”, thereby indicating that the firstcatalyst 53 has not degraded.

In this manner, if the mean maximum oxygen storage amount CSCmax is lessthan or equal to the first catalyst degradation criterion CSCRave (i.e.,if the first catalyst 53 is in a state where it should be determinedthat the first catalyst 53 has degraded), the CPU 71 determines that thefirst catalyst 53 has degraded provided that no abnormality of theupstream-most air-fuel ratio sensor 66 is detected. If an abnormality ofthe upstream-most air-fuel ratio sensor 66 has been detected in thatcase, the CPU 71 avoids determining that the first catalyst 53 hasdegraded. If the mean maximum oxygen storage amount CSCmax is greaterthan the first catalyst degradation criterion CSCRave (i.e., if thefirst catalyst 53 is in a state where it should be determined that thefirst catalyst 53 has not degraded), the CPU 71 determines that thefirst catalyst 53 has not degraded, regardless of whether an abnormalityof the upstream-most air-fuel ratio sensor 66 has been detected.

Subsequently, the CPU 71 proceeds to step S1724, in which the CPU 71stores the mean value of the maximum oxygen storage amount CUFmax4 andthe maximum oxygen storage amount CUFmax6 as a mean maximum oxygenstorage amount CUFmax of the second catalyst 54. Subsequently in stepS1726, the CPU 71 determines whether the mean maximum oxygen storageamount CUFmax is less than or equal to the second catalyst degradationcriterion CUFRave. If the mean maximum oxygen storage amount CUFmax isless than or equal to the second catalyst degradation criterion CUFRave,the CPU 71 proceeds to step S1728, in which the CPU 71 determineswhether the value of the upstream-most air-fuel ratio sensor abnormalityflag XSENR is “0”.

If it is determined in step S1728 that the value of the upstream-mostair-fuel ratio sensor abnormality flag XSENR is “0”, the CPU 71 sets thevalue of a second catalyst degradation determination result flag XUFR at“1” in step S1730, thereby indicating that the second catalyst 54 hasdegraded. Conversely, if it is determined in step S1728 that the valueof the upstream-most air-fuel ratio sensor abnormality determinationresult flag XSENR is not “0” (i.e., if the value is “1”), the CPU 71sets the value of the second catalyst degradation determination resultflag XUFR at “2” in step S1732, thereby indicating that thedetermination regarding degradation of the second catalyst 54 has notbeen performed.

If it is determined in step S1726 that the mean maximum oxygen storageamount CUFmax is greater than the second catalyst degradation criterionCUFRave, the CPU 71 sets the value of the second catalyst degradationdetermination result flag XUFR at “0” in step S1734, thereby indicatingthat the second catalyst 54 has not degraded.

In this manner, if the mean maximum oxygen storage amount CUFmax is lessthan or equal to the second catalyst degradation criterion CUFRave (ifthe second catalyst 54 is a state where it should be determined that thesecond catalyst 54 has degraded), the CPU 71 determines that the secondcatalyst 54 has degraded provided that no abnormality of theupstream-most air-fuel ratio sensor 66 is detected. If an abnormality ofthe upstream-most air-fuel ratio sensor 66 has been detected in thatcase, the CPU 71 avoids determining that the second catalyst 54 hasdegraded. If the mean maximum oxygen storage amount CUFmax is greaterthan the second catalyst degradation criterion CUFRave (i.e., if thesecond catalyst 54 is in a state where it should be determined that thesecond catalyst 54 has not degraded), the CPU 71 determines that thesecond catalyst 54 has not degraded, regardless of whether anabnormality of the upstream-most air-fuel ratio sensor 66 has beendetected.

Subsequently, the CPU 71 proceeds to step S1736, in which the CPU 71determines whether the sum of the mean maximum oxygen storage amountCSCmax, that is, a value regarding the maximum oxygen storage amount ofthe first catalyst 53, and the mean maximum oxygen storage amountCUFmax, that is, a value regarding the maximum oxygen storage amount ofthe second catalyst 54, is less than or equal to the entire catalystdegradation criterion CRave. If the aforementioned sum is less than orequal to the entire catalyst degradation criterion CRave, the CPU 71proceeds to step S1738, in which the CPU 71 determines whether the valueof the upstream-most air-fuel ratio sensor abnormality determinationresult flag XSENR is “0”.

If it is determined in step S1738 that the value of the upstream-mostair-fuel ratio sensor abnormality determination result flag XSENR is“0”, the CPU 71 sets the value of an entire catalyst degradationdetermination result flag XALLR at “1” in step S1740, thereby indicatingthat the combination of the first catalyst 53 and the second catalyst 54has degraded as a whole. Conversely, if it is determined in step S1738that the value of the upstream-most air-fuel ratio sensor abnormalitydetermination result flag XSENR is not “0” (i.e., if the value is “1”),the CPU 71 sets the value of the entire catalyst degradationdetermination result flag XALLR at “2” in step S1742, thereby indicatingthat determination regarding degradation of the first catalyst 53 andthe second catalyst 54 combined has not been performed.

Conversely, if it is determined in step S1736 that the aforementionedsum is greater than the entire catalyst degradation criterion CRave, theCPU 71 sets the value of the entire catalyst degradation determinationresult flag XALLR at “0” in step S1744, thereby indicating that theentire device of the first catalyst 53 and the second catalyst 54 hasnot degraded.

In this manner, if the aforementioned sum is less than or equal to theentire catalyst degradation criterion CRave (i.e., if the first catalyst53 and the second catalyst 54 are in a state where it should bedetermined that the entire device of the first and second catalysts 53,54 has degraded), the CPU 71 determines that the entire device of thefirst catalyst 53 and the second catalyst 54 has degraded provided thatno abnormality of the upstream-most air-fuel ratio sensor 66 has beendetected. If an abnormality of the upstream-most air-fuel ratio sensor66 has been detected, the CPU 71 avoids determining that the entiredevice of the first and second catalysts 53, 54 has degraded. If theaforementioned sum is greater than the entire catalyst degradationcriterion CRave (i.e., if the first catalyst 53 and the second catalyst54 are in a state where it should be determined that the entire deviceof the first and second catalysts 53, 54 has not degraded), the CPU 71determines that the first catalyst 53 and the second catalyst 54 havenot degraded as a whole, regardless of whether an abnormality of theupstream-most air-fuel ratio sensor 66 has been detected.

Subsequently, the CPU 71 proceeds to step S1746, in which the CPU 71sets all the values of the oxygen storage amounts OSA3, OSA4, OSA5, OSA6at “0”. Subsequently in step S1748, the CPU 71 stores the sum of themean maximum oxygen storage amount SCSmax and the mean maximum oxygenstorage amount CUFmax as a maximum oxygen storage amount Cmaxall of theentire catalyst device into the backup RAM 74. Subsequently in stepS1795, the CPU 71 temporarily ends the routine.

As described above, according to the catalyst degradation determiningmethod of the catalyst degradation determining apparatus of theinvention, the downstream-of-first catalyst air-fuel ratio sensor 67 andthe downstream-of-second catalyst air-fuel ratio sensor 68 are disposeddownstream of the first catalyst 53 and the second catalyst 54,respectively. Therefore, it is possible to reliably detect the time whenthe amount of oxygen stored in either one of the catalysts reaches “0”or the maximum oxygen storage amount. As a result, the maximum oxygenstorage amount CSCmax of the first catalyst 53 and the maximum oxygenstorage amount CUFmax of the second catalyst 54 can be determined withgood precision. Therefore, it is possible to determine whether the firstcatalyst 53 has degraded and whether the second catalyst 54 hasdegraded, separately, with good precision. The first catalyst 53 and thesecond catalyst 54 can be viewed as a single catalyst device. The methodallows determination as to whether the entire catalyst device hasdegraded.

During execution of the determination regarding catalyst degradation,the second lean air-fuel ratio is changed in accordance with the valueof the entire catalyst oxygen storage amount Cmaxall estimated duringthe previous determination regarding catalyst degradation, and is set soas to always equal an air-fuel ratio that is near the upper limit valueof the window range of the catalyst device occurring at the present time(see time t2 to t3, and time t6 to t7 in FIG. 5).

The reason for setting the second lean air-fuel ratio at an air-fuelratio that is near the present upper limit value of the window range ofthe catalyst device is as follows. That is, when the output Voxs2 of thedownstream-of-second catalyst air-fuel ratio sensor 68 changes from avalue indicating an air-fuel ratio rich of the stoichiometric air-fuelratio to a value indicating an air-fuel ratio lean of the stoichiometricair-fuel ratio (see time points t3, t7 in FIG. 5) as a mixture of thesecond lean air-fuel ratio flows into the first and second catalysts 53,54, the gas of the second lean air-fuel ratio fills a space defined bythe catalysts 53, 54 and the exhaust passages 51, 52 extending from theexhaust port 34 of the engine 10 to the downstream-of-second catalystair-fuel ratio sensor 68. At that time, the oxygen storage amounts ofthe first and second catalysts 53, 54 have reached the maximum oxygenstorage amounts CSCmax, CUFmax. Therefore, the oxygen storing functionsof the first and second catalysts 53, 54 are not effective, so thatnitrogen oxides NOx are likely to be emitted.

Therefore, if the second lean air-fuel ratio is a lean air-fuel ratiothat is considerably higher than the upper limit value of the windowrange of the catalyst device, a large amount of nitrogen oxides NOx willbe contained in the gas contained in the aforementioned space. Besides,the oxygen storing functions of the first and second catalysts 53, 54are not effective, and the efficiency of removal of nitrogen oxides NOxbased on the reducing functions of the first and second catalysts 53, 54has become low. Therefore, a large amount of nitrogen oxides NOx will bereleased into the atmosphere immediately after the time of a lean-sideswitch of the output of the downstream-of-second catalyst air-fuel ratiosensor (see time points t3, t7 in FIG. 5).

In contrast, if the second lean air-fuel ratio is always kept at anair-fuel ratio that is near the present upper limit value of the windowrange of the catalyst device as in the foregoing embodiments, theefficiency of removal of nitrogen oxides NOx based on the reducingfunction of the catalyst device immediately after the time of alean-side switch of the output of the downstream-of-second catalystair-fuel ratio sensor is kept at or above a predetermined high value,and the amount of nitrogen oxides NOx emitted immediately after theaforementioned switch can be minimized. Furthermore, since the secondlean air-fuel ratio is set at an air-fuel ratio that is as remote fromthe stoichiometric air-fuel ratio as possible, the length of time (timet2 to t3 and time t6 to t7 in FIG. 5) needed to bring the oxygen storageamount of the second catalyst 54 to the maximum oxygen storage amountcan be reduced, in comparison with the case where the second leanair-fuel ratio is pre-set at a lean air-fuel ratio close to thestoichiometric air-fuel ratio. The time (time t1 to t7 in FIG. 5) neededfor calculation of the maximum oxygen storage amount can also beshortened.

Still further, the first lean air-fuel ratio is set at an air-fuel ratiothat is on the leaner side of the second lean air-fuel ratio (anair-fuel ratio slightly above the upper limit value of the window rangethe catalyst device in a brand-new condition) (see time t1 to t2 andtime t5 to t6 in FIG. 5). The reason for setting the first lean air-fuelratio at an air-fuel ratio that is lean of the second lean air-fuelratio is as follows.

That is, during the period during which the air-fuel ratio of gasupstream of the first catalyst is controlled to the first lean air-fuelratio, the amount of oxygen stored in the first catalyst 53 does notreach the oxygen storage amount CSCmax prior to the end of the periodalthough the oxygen storage amount increases with elapse of time. Untilthe end of the period, that is, the time point at which the output ofthe downstream-of-first catalyst air-fuel ratio sensor 67 comes toindicate an air-fuel ratio lean of the stoichiometric air-fuel ratio, alean air-fuel ratio gas does not start to flow out of the first catalyst53, and therefore, the oxygen storage amount of the second catalyst 54remains at “0”.

Therefore, even if the first lean air-fuel ratio is set at an air-fuelratio that is higher by a certain amount than the upper limit value ofthe window range of the catalyst device, nitrogen oxides that flow intothe first catalyst 53 are removed due to the oxygen storing function ofthe first and second catalysts 53, 54 and therefore are not emitted intothe atmosphere during a period during which the air-fuel ratio of gasupstream of the first catalyst is controlled to the first lean air-fuelratio. Therefore, if the first lean air-fuel ratio is set at an air-fuelratio that is lean of the second lean air-fuel ratio as in the foregoingembodiments, the length of time (time t1 to t2, and time t5 to t6 inFIG. 5) needed to bring the amount of oxygen stored in the firstcatalyst 53 to the maximum oxygen storage amount can be reduced, incomparison with the case where the first lean air-fuel ratio is setequal to the second lean air-fuel ratio. The time (time t1 to t7 in FIG.5) needed for calculation of the maximum oxygen storage amount can befurther shortened.

Similarly, during an execution of the determination regarding catalystdegradation, the second rich air-fuel ratio is changed in accordancewith the value of the maximum oxygen storage amount Cmaxall of theentire catalyst estimated during the previous execution of thedetermination regarding catalyst degradation, so that the second richair-fuel ratio is always at an air-fuel ratio that is close to thepresent lower limit value of the window range of the catalyst device(see time t4 to t5 in FIG. 5).

The reason for always setting the second rich air-fuel ratio at anair-fuel ratio close to the present lower limit value of the windowrange of the catalyst device is as follows. That is, when the outputVoxs2 of the downstream-of-second catalyst air-fuel ratio sensor 68changes from a value indicating an air-fuel ratio lean of thestoichiometric air-fuel ratio to a value indicating an air-fuel ratiorich of the stoichiometric air-fuel ratio (see time point t5 in FIG. 5)as a mixture of the second rich air-fuel ratio flows into the first andsecond catalysts 53, 54, the gas of the second rich air-fuel ratio fillsa space defined by the catalysts 53, 54 and the exhaust passages 51, 52extending from the exhaust port 34 of the engine 10 to thedownstream-of-second catalyst air-fuel ratio sensor 68. At that time,the amounts of oxygen stored in the first and second catalysts 53, 54are both “0”, and therefore the oxygen releasing functions of the firstand second catalysts 53, 54 are not effective, so that unburnedcomponents, such as CO, HC, etc., are likely to be emitted.

Therefore, if the second rich air-fuel ratio is a rich air-fuel ratiothat is considerably lower than the lower limit value of the windowrange of the catalyst device, a large amount of CO and HC will becontained in the gas contained in the aforementioned space. Besides, theoxygen releasing functions of the first and second catalysts 53, 54 arenot effective, and the efficiency of removal of CO and HC based on theoxidizing functions of the first and second catalysts 53, 54 has becomelow. Therefore, a large amount of CO and HC will be released into theatmosphere immediately after the time of a rich-side switch of theoutput of the downstream-of-second catalyst air-fuel ratio sensor (seetime point t5 in FIG. 5).

In contrast, if the second rich air-fuel ratio is always kept at anair-fuel ratio that is near the present lower limit value of the windowrange of the catalyst device as in the foregoing embodiments, theefficiency of removal of CO and HC based on the oxidizing function ofthe catalyst device immediately after the time of a rich-side switch ofthe output of the downstream-of-second catalyst air-fuel ratio sensor iskept at or above a predetermined high efficiency value, and the amountof CO and HC emitted immediately after the aforementioned switch can beminimized. Furthermore, since the second rich air-fuel ratio is set atan air-fuel ratio that is as remote from the stoichiometric air-fuelratio as possible, the length of time (time t4 to t5 in FIG. 5) neededto consume the amount of oxygen stored in the second catalyst 54 to thelevel of “0” can be reduced, in comparison with the case where thesecond rich air-fuel ratio is pre-set at an air-fuel ratio close to thestoichiometric air-fuel ratio. The time (time t1 to t7 in FIG. 5) neededfor calculation of the maximum oxygen storage amount can also beshortened.

Furthermore, the first rich air-fuel ratio is set at an air-fuel ratiothat is on the richer side of the second rich air-fuel ratio (anair-fuel ratio slightly below the lower limit value of the window rangethe catalyst device in a brand-new condition) (see time t3 to t4 in FIG.5). The reason for setting the first rich air-fuel ratio at an air-fuelratio that is rich of the second rich air-fuel ratio is as follows.

That is, during the period during which the air-fuel ratio of gasupstream of the first catalyst is controlled to the first rich air-fuelratio, the amount of oxygen stored in the first catalyst 53 does notreach “0” prior to the end of the period although the oxygen storageamount decreases with elapse of time. Until the end of the period, thatis, the time point at which the output of the downstream-of-firstcatalyst air-fuel ratio sensor 67 comes to indicate an air-fuel ratiorich of the stoichiometric air-fuel ratio, a rich air-fuel ratio gasdoes not start to flow out of the first catalyst 53, and therefore, theoxygen storage amount of the second catalyst 54 remains at the maximumoxygen storage amount CUFmax.

Therefore, even if the first rich air-fuel ratio is set at an air-fuelratio that is lower by a certain amount than the lower limit value ofthe window range of the catalyst device, CO and HC that flow into thefirst catalyst 53 are removed due to the oxygen releasing functions ofthe first and second catalysts 53, 54 and therefore are not emitted intothe atmosphere during a period during which the air-fuel ratio of gasupstream of the first catalyst is controlled to the first rich air-fuelratio. Therefore, if the first rich air-fuel ratio is set at an air-fuelratio that is on the richer side of the second rich air-fuel ratio as inthe foregoing embodiments, the length of time (time t3 to t4 in FIG. 5)needed to consume the amount of oxygen stored in the first catalyst 53to the level of “0” can be reduced, in comparison with the case wherethe first rich air-fuel ratio is set equal to the second rich air-fuelratio. The time (time t1 to t7 in FIG. 5) needed for calculation of themaximum oxygen storage amount can be further shortened.

Even in a case where degradation of the upstream-most air-fuel ratiosensor 66 has progressed and an abnormality of the upstream-mostair-fuel ratio sensor 66 has been detected, it is possible to determinethat neither one of the first and second catalysts 53, 54 has degradedand that the catalyst device has not degraded. As long as it isdetermined that each catalyst has not degraded, there is no need toreplace the upstream-most air-fuel ratio sensor 66. Thus, the time ofreplacing the upstream-most air-fuel ratio sensor 66 can be delayed.Furthermore, as long as an abnormality of the upstream-most air-fuelratio sensor 66 is detected, determination that a catalyst has degradedis avoided. Thus, a false determination regarding degradation of eachcatalyst can be prevented.

The invention is not limited to the foregoing embodiments, but may bemodified in various manners within the scope of the invention. Forexample, although in the foregoing embodiments, the first rich air-fuelratio, the second rich air-fuel ratio, the first lean air-fuel ratio andthe second lean air-fuel ratio are constant during the correspondingmodes, the air-fuel ratios may be variable.

Furthermore, in the foregoing embodiments, the second lean air-fuelratio and the second rich air-fuel ratio are changed in accordance withthe value of the maximum oxygen storage amount Cmaxall of the entirecatalyst estimated during the previous execution of the determinationregarding catalyst degradation, that is, a degradation index. As forsuch a degradation index, however, it is possible to adopt the value ofthe maximum oxygen storage amount CSCmax of the first catalyst 53, thevalue of the maximum oxygen storage amount CUFmax of the second catalyst54, or a value obtained by weighting the value of the maximum oxygenstorage amount CSCmax of the first catalyst 53 and the value of themaximum oxygen storage amount CUFmax of the second catalyst 54 in apredetermined weighting fashion, and summing the weighted values, sothat the second lean air-fuel ratio and the second rich air-fuel ratiomay be changed in accordance with such an index value.

For example, if the value of the maximum oxygen storage amount CUFmax ofthe second catalyst 54 is adopted as a degradation index, it is possibleto set the second lean air-fuel ratio at an air-fuel ratio close to thepresent upper limit value of the window range of the second catalyst 54,the window range narrowing in accordance with the degree of degradationof the second catalyst 54, and set the second rich air-fuel ratio at anair-fuel ratio close to the present lower limit value of the windowrange of the second catalyst 54, and set the first lean air-fuel ratioat an air-fuel ratio slightly above the upper limit value of the windowrange of the second catalyst 54 in a brand new condition, and set thefirst rich air-fuel ratio at an air-fuel ratio slightly below the lowerlimit value of the window range of the second catalyst 54 in the brandnew condition.

Still further, in the foregoing embodiments, the second lean air-fuelratio and the second rich air-fuel ratio may be changed on the basis ofa value that changes in accordance with the temperature of the firstcatalyst 53 acquired by first catalyst temperature acquisition means(not shown) for acquiring the temperature of the first catalyst 53and/or the temperature of the second catalyst 54 acquired by secondcatalyst temperature acquisition means (not shown) for acquiring thetemperature of the second catalyst 54.

Although in the foregoing embodiments, the maximum oxygen storageamounts CSCmax, CUFmax of the first and second catalysts 53, 54 areestimated on the basis of the value of the output vabyfs of theupstream-most air-fuel ratio sensor 66, the maximum oxygen storageamounts CSCmax, CUFmax may be estimated on the basis of the value of aknown upstream-of-first catalyst air-fuel ratio that is constant duringeach mode if an abnormality of the upstream-most air-fuel ratio sensor66 has been detected. Specifically, for example, during the third mode(time t3 to t4) in FIG. 5, the upstream-of-first catalyst air-fuel ratiois the constant first rich air-fuel ratio (stoich/1.02). Therefore, themaximum oxygen storage amount CSCmax3 during the third mode may bedetermined as in 0.23×Imfr3×(stoich−abyfR1)×Δt3 based on theaforementioned mathematical expressions 1 and 2, where Δt3 is the lengthof time t3 to t4; abyfR1 is the first rich air-fuel ratio; and mfr3 isthe amount of fuel supplied per unit time during the third mode.

In the foregoing embodiments, if the catalyst degradation determiningcondition is fulfilled, the upstream-of-first catalyst air-fuel ratio isset at the first lean air-fuel ratio regardless of the then-occurringoutput Voxs1 of the downstream-of-first catalyst air-fuel ratio sensorand the then-occurring output Voxs2 of the downstream-of-second catalystair-fuel ratio sensor. However, in order to lessen emissions, it ispreferable that the upstream-of-first catalyst air-fuel ratio initiallyset for estimation of the oxygen storage amount be variably set inaccordance with the output Voxs1 of the downstream-of-first catalystair-fuel ratio sensor and the output Voxs2 of the downstream-of-secondcatalyst air-fuel ratio sensor that occur at the time of fulfillment ofthe catalyst degradation determining condition.

More specifically, if both the downstream-of-first catalyst air-fuelratio sensor output Voxs1 and the downstream-of-second catalyst air-fuelratio sensor output Voxs2 indicate rich air-fuel ratios when thecatalyst degradation determining condition is fulfilled, the control ofthe upstream-of-first catalyst air-fuel ratio is started in the firstmode. That is, the upstream-of-first catalyst air-fuel ratio is set atthe first lean air-fuel ratio.

If the downstream-of-first catalyst air-fuel ratio sensor output Voxs1indicates a lean air-fuel ratio and the downstream-of-second catalystair-fuel ratio sensor output Voxs2 indicates a rich air-fuel ratio atthe time of fulfillment of the catalyst degradation determiningcondition, the control is started in the second mode in which theupstream-of-first catalyst air-fuel ratio is set at the second leanair-fuel ratio.

If both the downstream-of-first catalyst air-fuel ratio sensor outputVoxs1 and the downstream-of-second catalyst air-fuel ratio sensor outputVoxs2 indicate lean air-fuel ratios at the time of fulfillment of thecatalyst degradation determining condition, the control is started inthe third mode in which the upstream-of-first catalyst air-fuel ratio isset at the first rich air-fuel ratio. In this case, the maximum oxygenstorage amounts estimated during the initial third mode and thesubsequent fourth mode are not accurate, and therefore should preferablynot be used for determination regarding catalyst degradation. In apreferable method, the sequence of the third mode and the fourth mode isperformed again after the end of the sixth mode in order to measure themaximum oxygen storage amounts in the third and fourth modes, and thethus-measured maximum oxygen storage amounts are used for determinationregarding catalyst degradation. In this case, the maximum oxygen storageamount of the first catalyst 53 acquired in the initial fifth modecorresponds to a first catalyst's first maximum oxygen storage amount;the maximum oxygen storage amount of the second catalyst 54 acquired inthe following fifth mode corresponds to a second catalyst's firstmaximum oxygen storage amount; the maximum oxygen storage amount of thefirst catalyst 53 acquired in the following third mode corresponds to afirst catalyst's second maximum oxygen storage amount; and the maximumoxygen storage amount of the second catalyst 54 acquired in thefollowing fourth mode corresponds to a second catalyst's second maximumoxygen storage amount.

Then, it may be determined whether the first catalyst 53 has degraded onthe basis of the first catalyst's first maximum oxygen storage amountand the first catalyst's second maximum oxygen storage amount (e.g., onthe basis of a mean value of the two amounts), and it may be determinedwhether the second catalyst 54 has degraded on the basis of the secondcatalyst's first maximum oxygen storage amount and the second catalyst'ssecond maximum oxygen storage amount (e.g., on the basis of a mean valueof the two amounts). It is also possible to determine whether a catalystdevice of the first catalyst 53 and the second catalyst 54 combined hasdegraded on the basis of the aforementioned four maximum oxygen storageamounts (e.g., on the basis of a mean value thereof).

If the downstream-of-first catalyst air-fuel ratio sensor output Voxs1indicates a rich air-fuel ratio and the downstream-of-second catalystair-fuel ratio sensor output Voxs2 indicates a lean air-fuel ratio atthe time of fulfillment of the catalyst degradation determiningcondition, the control is started in the fourth mode in which theupstream-of-first catalyst air-fuel ratio is set at the second richair-fuel ratio. In this case, the oxygen storage amounts estimated inthe initial fourth mode are not accurate. Therefore, a construction isprovided for avoiding the use of these oxygen storage amounts fordetermination regarding catalyst degradation. It is preferable toprovide a construction in which the third and fourth modes are performedafter execution of the fifth and sixth modes, and these maximum oxygenstorage amounts estimated in the third and fourth modes are used fordetermination regarding catalyst degradation. In this case (where thedownstream-of-first catalyst air-fuel ratio sensor output Voxs1indicates a rich air-fuel ratio and the downstream-of-second catalystair-fuel ratio sensor output Voxs2 indicates a lean air-fuel ratio atthe time of fulfillment of the catalyst degradation determiningcondition), it is also possible to adopt a construction in which thecontrol is started with the first rich air-fuel ratio, and the fifth,sixth, third and fourth modes are sequentially performed to determinemaximum oxygen storage amounts, starting at the time point of a switchof the downstream-of-second catalyst air-fuel ratio sensor output Voxs2from a lean air-fuel ratio-indicating value to a rich air-fuelratio-indicating value.

While the invention has been described with reference to preferredembodiments thereof, it is to be understood that the invention is notlimited to the preferred embodiments or constructions. To the contrary,the invention is intended to cover various modifications and equivalentarrangements. In addition, while the various elements of the preferredembodiments are shown in various combinations and configurations, whichare exemplary, other combinations and configurations, including more,less or only a single element, are also within the spirit and scope ofthe invention.

1. A catalyst degradation determining method for use with an emissioncontrol apparatus of an internal combustion engine that includes: acatalyst disposed in an exhaust passage of the internal combustionengine, and a downstream-of-catalyst air-fuel ratio sensor disposed inthe exhaust passage downstream of the catalyst, the method comprisingthe steps of: acquiring an oxidizing-reducing capability index valuethat changes in accordance with a degree of an oxidizing-reducingcapability of the catalyst; controlling an upstream-of-catalyst air-fuelratio occurring upstream of the catalyst to an air-fuel ratio that islean of a stoichiometric air-fuel ratio so that the catalyst storesoxygen in the catalyst up to a maximum storage amount of oxygen; thencontrolling the upstream-of-catalyst air-fuel ratio to a rich air-fuelratio that is rich of the stoichiometric air-fuel ratio and that has avalue that is determined in accordance with the oxidizing-reducingcapability index value, until a time point when an output of thedownstream-of-catalyst air-fuel ratio sensor indicates an air-fuel ratiothat is rich of the stoichiometric air-fuel ratio; estimating a maximumoxygen storage amount of the catalyst by taking into account the valueof the rich air-fuel ratio to which the upstream-of-catalyst air-fuelratio was controlled; and determining whether the catalyst has degradedbased on the estimated maximum oxygen storage amount of the catalyst. 2.The catalyst degradation determining method according to claim 1,wherein: the emission control apparatus of the internal combustionengine with which the catalyst degradation determining method is usedincludes an upstream-of-catalyst air-fuel ratio sensor disposed in theexhaust passage upstream of the catalyst, and an upstream-of-catalystair-fuel ratio sensor abnormality detector that detects an abnormalityof the upstream-of-catalyst air-fuel ratio sensor, the maximum oxygenstorage amount of the catalyst is estimated based on an output of theupstream-of-catalyst air-fuel ratio sensor, a determination that thecatalyst has degraded is prohibited in a case where the catalyst is in astate in which it is to be determined that the catalyst has degradedbased on the estimated maximum oxygen storage amount, and where anabnormality of the upstream-of-catalyst air-fuel ratio sensor has beendetected, and it is determined that the catalyst has not degradedregardless of whether an abnormality of the upstream-of-catalystair-fuel ratio sensor has been detected, in a case where the catalyst isin a state in which it is determined that the catalyst has not degradedbased on the estimated maximum oxygen storage amount.